He's just so relieved to see her safe and alive and unhurt, honestly that's it, there's no other reason for the kiss, no other reason for the human contact outside their norm of one-armed hugs and shoulder clasps. She's safe, that's all, this girl that he's put so much time and energy into, this girl that he loves.
He didn't mean it to deepen, didn't mean any of that to happen. He didn't realize she would kiss him back, that it would turn into something it shouldn't and couldn't be, that anything would happen, because he didn't intend for anything to happen.
Because it's Kel, you can't do this with Kel, he keeps thinking over and over, even as he does.