Why is my guy friend mad at me?? Guys, help me out here.

One of my really close friends at work is a guy and lately he has been acting like he hates my guts and I can't really tell why. We haven friends for about a year and a half and we go for coffee and lunch all the time and we can talk about pretty much everything. Unfortunately, I work a lot outside of work so I'm always really busy. When he asks me to do stuff outside of work I never really have the time so I can't go. Maybe this is why he's mad at me?

My girlfriends keep telling me he most likely likes me but the way he treats me is like one of the guys. Also, there's this other guy in the office that flirts with me a lot but how can that p*ss him off?

Normally, I wouldn't care when someone is mad at me for no reason but I would hate to lose him as a friend.

Can you guys shed some light on this for me?