Why am I always sick?!

I'm a healthy individual. I'm in college, male, 18, I have no known medical conditions, I run, work out, play lacrosse, eat healthy, take vitamins, wash my hands, bathe regularly, don't share cutlery or cups, don't drink, don't smoke, everything. Today wraps up the 6th week I've been sick this school year. Sinus infection after sinus infection, now I have another sinus infection, and my tonsils are the size of golfballs. It's not normal or healthy for someone to be sick this much. All my colds last 2-3 weeks, when it seems like everyone else is only every sick once a year and only for a few days. Do I have some sort of condition? What am I doing wrong? How can I help my immune system?