Yea I can't put on weight either, but I don't really care much, the doctor says I'm healthy so no worries.
drink water! mind over everything
Because it is physically harder for some people to lose weight.
Having the will to lose weight is only half the battle, you have to also have a decent metabolism, eat a decent diet, and have the time to invest in exercising, which gets harder and harder as you get older.
I can't speak from experience here, because it's tremendously easy for me to lose weight; I wrestled in high school for 4 years and there were times when I had to lose ridiculous amounts of weight in short amounts of time. For senior year, I cut down from 155 lbs to 125 in about two and a half weeks. At about 5'10'' I was losing more muscle than anything at that point.