Why do i have such a weak immune system?

So about 2 years ago I got my tonsils removed because they were constantly swollen so i always had a sore throat. After getting my tonsils removed, i have been getting ear infections quite frequently! I used to get them when i was younger but they went away, when i started having the tonsil problems. I'm 20 and I feel like i'm too old to get so many ear infections. The doctor said i have scar tissue in my ear, is that really serious? Also, about a year ago i had a cold which turned into pneumonia. I had rested and drank alot of fluids prior to getting it but it still got worse. It seems like whenever i have a cold now it hangs around forever and turns serious. I started getting a cold two weeks ago and for the last week or so i have had a cough that produces green phlegm. I don't have a fever so i haven't gone to the doctor, but i feel like i'm getting pneumonia again. I eat healthy, get plenty of sleep, exercise, but i can't fight off the simplest cold. Why not?