Because we haven’t yet learned how to not dump our feelings of loneliness all over someone.
You need to realize that if you’ve “fallen in love” with someone, that feeling doesn’t come from a place of healthy mutual attraction. You don’t really even know that person, otherwise you’d know why they don’t love you back.
We do this because we’re afraid of rejection. We don’t properly communicate our feelings to each other because we’re worried how they might react. Sometimes for good reason, sometimes for not so good reason.
You’ve just latched on to someone who seems like they could alleviate your loneliness without ever really even getting to know them. We want them to “love us back” before we can even get started getting to know them.
Forget about love for now. Just talk to people. When you find one you’re attracted to, tell them you’re attracted to them. They’re either going to return those feelings or you’ll get some feedback. If it’s feedback, take it to heart and try again with someone else. If you don’t have that many people you’re attracted to, then you need to get out more.
Mutual attraction can be difficult to find, but you definitely will never find it if you don’t communicate.