Because he's hot, talented and just... fucking awesome.
[after Sam's OPR interview]
Samantha: He asked me about us. He had all this stuff. Um, security card access records of nights that we left together; phone logs.
Jack: And what did you say?
Samantha: I told him it wasn't true.
Jack: You have no idea of what he knows or doesn't know. That's his ace up the sleeve - that's how he wins.
Samantha: I was just trying to protect you.
Jack: Yeah, but you didn't have to lie. Even if he had hard evidence, there's nothing he could do about it.
Samantha: We broke at least half a dozen conduct codes.
Jack: And lying to an OPR investigator could get you fired.
Samantha: And sleeping with an agent under your command could get you fired. I'm sorry I lied. I was just trying to save your ass.