I dont get why my hair is falling out

[sheamie95]

I have a very bad receding hair line now at the age of 18 and I am beginning to now notice diffuse thinning. I first noticed this about 3 years ago when I had a very mild receding hairline at the time. I dont see how it could be genetic for me. My mothers two brothers are in their thirties and have full heads of hair and my mothers father is 68 and also has an excellent head of hair. My fathers father had a full head of hair at the age of 77 when he died. My dad and my dads two brothers are in their mid to late fifties now and have had full heads of hair all their lives but my dad and his two brothers have thinned only over the least few years but neither are bald just a reasonable sized spot on the vertex not too big or too small which does tend to come with age. I dont get why its happening to me so young. Both sides of the family have a good history of head hair and also neither of my cousins are thinning either. What is really going on?