Let's break both scenarios down.
In the first case, people might be involved with people they don't love for several reasons:
- They're with them for their money
- They were on the rebound after their last relationship ended in failure
- They don't want to be alone
- They have no other options at the moment
- They're trying to please their parents and/or friends
- For purely physical reasons
And here are some reasons why a person may love someone, but can't be romantically involved with them:
- He or she is already taken
- The person doesn't have the nerve to ask them out or try to take the friendship to the next level
- They don't want to jeopardize the friendship they already have
- The individual doesn't feel he or she is "good enough" for the other (i.e., they're out of the person's league)
- The other person doesn't want a serious relationship right now
Both scenarios can be a real bummer. It's hard to pinpoint what's worse: being with someone you don't love, or being unable to be with the one you do. Either way, the person finds himself wholly unhappy.
Life is too short to be in either situation. We should all strive to find real love -- someone with whom we're truly compatible. If you get stuck with someone you're truly not happy with, it can only lead to bitterness, resentment, and possibly cheating.
If you have found someone you truly love, don't keep such feelings bottled up. Express how you feel sooner than later. If, after making a conscious effort to woo the object of your desire, you realized there's really no hope, then concede you did your best and move on to someone who can reciprocate your feelings and efforts.
Be open to the possibility that your situation can change and it will. But you have to do your part to make it a reality.