Why does my hair break off so much?

If I do not use a anti-breakage shampoo for my hair then it breaks off usually in large amounts (if combed), but even when I am just sitting there and I move my hair or anything strands of my hair will flutter onto my shirt? What causes this and is there any NATURAL cures? Also I wash my hair in cold water and blow dry it and straighten it 5 out of 7 days a week which I am pretty sure causes the breakage.