Why some people are so possessive towards their partners

They may have low self-esteem by feeling that the partner may leave them one day for someone better. It also could be that the partner may have trust issues.

The way they process trauma. It's a flaw.

Insecurity, jealous, or just immature possibly? Could be a lot of reasons

Because they're mine, not yours.

Fear of losing them?

Why not?

Pffffft

Some day you’ll date a real man and he will be nothing of what you describe