WARNING: Spoilers through S10.
If you look at averages, you can't help but see Daniel definitely has the worst track record on SG-1 for those 'bad for you' attractions. (And, for this discussion, an 'attraction' is being defined as not just sparkage, but enough connection going on to at least genererate an on-screen kiss.) Going by what we've seen, Daniel's really only had one solid relationship--Sha're. Then there are all the bad ones.
While Sam can match a couple of "really bad" attractions (her engagement to Jonas Hanson, who is not one of the good guys and since she was engaged, we have to go with there being some attraction at some point, and Fifth's who openly declares his love for Sam), Daniel can meet those two with the very self-centered and Daniel destructive Shyla, and then with Hathor. And then he can add in Linea/Kera. And there's Vala who when she was being very bad was also very attracted. We also can't leave Adria out--she may be the most fatal of all the attractions. Once could be called back luck, but with at least four psychos in there who've come on to him, this is more like a track record.
So is it just the pretty that draws the bad girls--or something else? With that bad a record you have to wonder. (And, after Linea/Kera, it's actually not much of a wonder that there's a long time with Daniel not going near an emotional situation.) You also have to notice that he does set himself up for these things.
However, one thing here in Daniel's defense, is that while he pulls in this kind of attraction in, he's not drawn to it himself. With Hathor, Shyla, and even to an extent with Kera, he tries to resist, and tries to talk himself out of things--but these women just aren't listening.
With Hathor and Shyla, Daniel puts himself into it by stepping in and basically acting the good guy--the hero, the knight. For Hathor, he's the one who takes off her restraints, and thereby becomes her way into the base. She more than uses him. And while Hathor has to show up on Jack's list, too, of not good connections--there is that one embrace--Jack isn't the one who ends up being the sperm donor. With Shyla, there's about the same opening in that Daniel again has to act--he saves Shyla's life, and the thanks he gets for that is nutcase devotion and addiction to Goa'uld technology. This is not a balanced woman by any means.
With Linea/Kera, Daniel's still doing the rescue thing--she's in need of help. But this is also a case where he's flirting with her like crazy.
I know this bugs lots of folks, as in how could Daniel bury Sha're and then even notice someone else. But to me this makes a lot of sense. Grief takes people in different ways, and I've always had the belief that Hammond would require Daniel to take a leave of absence--so it's always seemed as if there would have been a few weeks between Sha're's death and this. Daniel is also someone who does struggle against the odds--it would make sense that he is trying to reconnect emotionally with himself, and with others. (And I think that what he was looking for here was a light, fun flirtation--something to just ease his own emotions back a little from the raw state of loss. But he got pulled in over his head, which is not all that uncommon for him.)
Daniel is quick to try and back out of what gets started by Kera. He warns her that he's on the rebound, he's not really fit for any kind of relationship He even starts to say that he doesn't trust himself right now--he doesn't finish the thought, but the implication is .that this would be more about using someone to ease his loss, than to make a real connection. He's hurting and not really up for more, but she pushes and he gives. And after it happens--and from Kera's body language, and how possessive she gets, I do believe they had sex--Daniel's the one who pulls back and away. His body langauge goes to pulling back, while she keeps pushing. So, for him, all the physical signs are that this was a mistake. And then it becomes an even bigger mistake when she regains her memories.
I do think a light flirtation would have been good for Daniel at this stage--it would have let him bring out feelings he's having trouble dealing with. He could have reconnected. Having this go to sex too soon is an issue, but then to have it turn out to be with someone like Linea--well, that's going to leave some scars, even in other circumstances. This is going to cement the idea that pretty much he had his one shot at love with Sha're and blew it.
But let's get back to the attractions part of these connections.
With Hathor, Shyla, and even Linea/Kera, Daniel does offer himself up to them. He's the one who steps out there to save them--trouble is, it's always a case of no good deed goes unpunished.
To balance the record, it should be noted that Daniel's habit of not being able to turn his back on someone doesn't always get him stuck with a psycho, and doesn't always end badly. In Demons, Mary gets her rescue--and since she has Simon, she also doesn't seem to want to cling to anyone else (and Daniel is the one here who won't turn his back on her need). Melocia in The Broca Divide also benefits from Daniel's chivalrous instincts--but there's no real connection made here for either of them. Sarah also benefits in that she's rescued, but that flame was out a long time ago.
Which brings us up to Vala--both a bad person, and yet good for Daniel in many ways. By the time she arrives, Daniel has learned not to be quite so trusting--he's more than willing to zat her, and trade punches. (And it's very nice that by S10 and Bounty, Daniel is more than willing to not let any woman pick him up--not even one met in a library--meaning Daniel either he does have some kind of good instincts about this, or he really doesn't date much at all.) But he does get suckered into Vala's story about needing to rescue her people--he still wants to believe in helping others. He believes in others.
And that may be the key here--it could be that that is what appeals, on a very core level, to even the worse of these women. We all want to believe in our own value--and Daniel offers up that up to others. It could well be that it's what these women see reflected back to them in his eyes. Shyla sees reflected back a person who is just lost--but who can be better. Hathor sees, in Daniel, someone she can use--someone with knowledge to use, but perhaps her host also sees someone who can offer not just worship, but a rare kindness. Linea/Kera sees an offer of acceptance--and she sees an equal to her own intelligence, a connection she may never have had before. Vala is both drawn back, and drawn into changing her ways to match what Daniel sees in her--what Daniel gives her in trusting his life to her. Even Adria, who wants to and does change Daniel, also can't resist the reflection she gets back of herself from him.
In Meridian Sam talks about the effect Daniel has on people--how Jonas had to take action to steal the Naquadria because of Daniel's actions, how she has changed because of him, too. How she sees what really matters--and that's because of him. Even the worse of us will believe we are good--we want to see that reflected back to us in those we hold close. It could be that simple--the bad ones are drawn to Daniel because he can still see the good, even in them. He can see the chance for more.