Why am I losing hair at the age of 17?

I'm a 17 year old male. I dunno why but I've lost a lot of hair recently and you can actually see my scalp. I had my board exams this year and I was under a lot of stress...so I think its because of this. After my exams got over my hair started to grow again. But my scalp is still slightly visible and when people take a closer look at me they say, " dude, you're losing hair ". When I was 15 I also had a lot of hair loss but after about 6 months it grew back and I actually had really thick long hair. My dad also experiences the same problem but I think it started for him when he reached 40. My mom's dad is 78 years old and he still has some hair on the back of his head and I think he started to lose hair when he was 60. So any ideas why this is happening to me?