Effort makes a task less attractive

Mental effort is costly, so we generally prefer to work on an simple task rather than a hard task. We procrastinate more if we expect a certain task to be hard.

This happens because the more effort a task requires, the more someone stands to gain by putting the same amount of effort into something else. This is called, in economical terms, opportunity cost. Opportunity costs make working on something that seems hard feel like a loss.