Why dandruff occurs ? how to get rid of it?..does it weaknes hair or cause haifall?

Dandruff: Dandruff can be caused by dry skin, irritated, oily skin (seborrheic dermatitis), not shampooing often enough, psoriasis, eczema, sensitivity to hair care products (contact dermatitis), or even a yeast-like fungus (malassezia). When regular shampoos fail, otc dandruff shampoos may succeed. But dandruff shampoos aren't all alike, and you may need to experiment until you find one that works for you.