Gamblers and fisherman sometimes have a difficult time controlling their need to gamble and fish, respectively, because

“Variable ratio” is the answer if I’m not wrong

Gamblers and fisherman sometimes have a difficult time controlling their need to gamble and fish, respectively, because of the Variable-ratio schedule of reinforcement.

Explanation:

A schedule of reinforcement is part of operant conditioning which states about instances when a behavior will be reinforced, whether with each time, or none at all.

Reinforcements (positive or negative) aim to strengthen a behavior. Variable-ration schedule is a part of partial reinforcement which leads response to be reinforced only at part of the time.  

Variable-ratio schedule occurs when a reinforcement results in a steady response rate with reinforcements after an unpredictable number of responses, like after 2, 4, 5, 9 and so on.

When a gambler gambles the first time, he/she may not win; the second time also none; so decides to quit and leave but finally decides to play one last time and wins at that time. This win reinforces him/her to gamble again. The same thing happens with fishermen also when they cannot fish for the first few attempts but finally succeeds which will make them to do fishing again.