How come I can gain muscle easily, but it takes time for me to lose fat?

I'm 190 lbs but everybody gets surprised when I tell them that. They think I'm 160 lbs. I'm in very good shape, I'm able to outrun most of my friends when we play manhunt lol. But the main problem is, I can gain muscle very easily, but it takes time for me to lose fat. I'm trying to cut myself up since its bathing suit season, and I want to know how I can lose this stubborn fat I'm carrying on my arms and my stomach as quickly as possible. I want my body to be made up of as much muscle mass as possible, and the least amount of fat as possible