The human brain is a pleasure-seeking machine, while always looking on the look-out to avoid 'displeasure'. For those with addictive tendencies, unfortunately, the brain usually goes with whatever activity brings the immediate/fast reward without thinking of the longer consequence (Heyman's Choice Theory that addicts are addicted by choice, and that the brain is radically altered by such behavior, in contrast to the disease model of addiction). People who are least likely to fall into addictive tendencies have brains which are keenly aware of long-term consequences of particular behaviors and will therefore hold-out on immediate gratification for a greater future payout, or lower likelihood of negative consequence.

On another note, it makes perfect sense how an abundance of dopamine in the brain will affect the brains activity and possibly its structure too (in much the same way a addict or gambler's brain is altered). BEHAVIORS CAN and DO alter the brain's structure, not just drugs.

It's pretty hard. Figuratively and Literally.