Because he was didn't want to be just another one of Tony's conquests, plus he was afraid; he wasn't sure what, exactly Tony wanted (friends with benefits or more?) and he didn't quite know what Tony was thinking. A little part of him was also concerned about his job, too. Tim just isn't an impulsive character, and so I figured in a situation like this he'd panic rather than go with the flow.

I wanted to keep the piece short, sweet, and simple as a first NCIS fic instead of trying to stick all this character development into it since I wasn't familiar with Tim and Tony at the time I wrote it, which is why I didn't really go into that.