Experience and Rice
Kevin Levin did not feel guilty for caring more about Gwens safety than the well-being of Tiny; hed been around aliens too long and around girls too little. He knew Ben meant for his joke to be, well, a joke, but Kevins response was genuine:
When he was in the Null Void, any females he mightve encountered were too like himself to be even in the least bit appealing- fighting for survival, plotting ways to get ahead. When he got back to Earth, girls took a back seat once more to his new, ahem, business. Then along came Tennyson and Tennyson. Truth be told, Kevin originally said those things to Gwen because he was trying to figure out a way to get ahead, just like always. She seemed so gullible and so damn caring that Kevin decided to tell her the things she probably wanted to hear. In the back of his mind, he was figuring out ways to lure her into his life and get back at Tennyson for all the things hed done to Kevin. Kevin did realize, of course, that it was really his own actions that caused his trapping in the Null Void, but still. The appearance of his old rival brought every feeling and every thought hed managed to get over in those five years back to the surface like a bubbling cauldron. But ironys a bitch and Gwen Tennyson didnt seem to be affected in the least by his (admittedly very crappy) attempts to gain her favor. Instead she focused on the bigger picture, the greater good, always. Ben was the same way, but while he was more focused on the glory, on it being his duty, his responsibility to help others, Gwen did it because she wanted to. She was never given any recognition for what she did, but she continued to do it just the same. By the time Gwen did seem to start caring about his comments, Kevin knew two things: first, that she could kick his butt if he tried anything. Itd be a hell of a fight, but in the end, he would probably let her win. Second, girls were confusing.