As a counselor and a parent, I would say that sometimes the child enjoys the activity, but other times they feel pressured to participate. This happens a lot when the father was involved in sports and he assumes that his kids will be athletic. There is also the notion that "real guys" should be involved in sports.
It is important to expose children to a variety of activities and hobbies, as some of these will become their passion and even lead to their life career. Without exposure the child may never come to realize that they like that particular activity.
If the child loses interest or seems reluctant to continue in the activity, it is wise to speak openly with him or her, and find out what's going through their head. There may be a problem with another child or the adult in charge, or your child may feel incompetent and intimidated.
Some kids are involved in too many afterschool activities and don't have time to rest, do their homework, and just be kids. It's important to keep their life balanced bewteen structured activities and free time.
The most important thing is to keep communication open so the child feels safe in sharing how they feel about what's going on inside.