I care about a friend of the opposite sex, but why doesn't she always return my messages?

Because she still thinks you're not important. Although stalking is a magic weapon for chasing girls, there are many successful examples. But personally, since she doesn't care, you'd better mind it. Love cannot be pursued. Caring is good, but it should be moderate and send less information, otherwise it will be considered harassment.

If he doesn't reply to you, you don't have to ask him. He may have gone to other things or gone on a business trip, and he won't be able to return your message for a while, so you should choose to wait until he returns to you. If he doesn't reply, you don't have to bother him anymore. Sometimes when he contacts you actively, you can reply to him actively, which is better and saves him trouble. Do what you have to do and don't want to contact him again.