The reasons I think he is interested :
He comes to my office at least several times a week, sometimes he comes to chat several times a day. When he first started with the company, he would come almost every day under the pretense of needing to borrow things or ask me questions, and now he comes just to talk about work or life. I share an office with two other people, who he rarely acknowledges. This has gone on for months, and we do not work on the same teams, so our business does not overlap in any context. We have gone to a few happy hours together and when those events got extended, he made it clear that he wasn't going to the extension if I wasn't going. He sits next to me or close to me at any and all events that we have both been at. We have exchanged phone numbers, but we don't talk much outside of work. If he is travelling for work, I usually hear from him regardless through our inner office messaging system. Other people seem to have noticed this in my office and have asked me how he is doing, etc. Net, he really seems to try to find ways to talk to me or be close to me.
The reasons I question if he is interested:
He talks to a lot of girls in our office. It doesn't seem to be the same frequency as me, but it still occurs. We are both single, we have started talking a little about online dating. He has now mentioned two dates to me, one was a few weeks ago and he has one tonight. Very brief mentions when we are discussing our weekend plans or funny things that relate to the world of dating.
The biggest thing for me is that I'm not sure he would mention other girls if he was interested.