Most Helpful Guy
Not interested. Distracted. Shy.
There are any number of reasons.
Keep trying I say.
Most Helpful Girl
It's hard to say without knowing the context of the situation. For example, where were you/what were you doing/how do you know him/how WELL do you know him/what kind of mood was he in? etc.
More than likely he was just feeling or serious or honestly, just not interested. Although, like I said before, that would be impossible to gauge based on your brief question.