Can any doc tell me what's premature ejaculation and why do men have it?

Don't ask "why": Manage it. A condom or a dab of a topical anesthetic on your glans may do the trick. Your physician can offer a very low dose of a selective serotonin reuptake inhibitor. Or you and your partner(s) can learn and practice the squeeze technique -- this is fun, will let you reprogram your nervous system to make you a superlatively excellent lover, and is your best long-term solution. Best wishes.