Ugh, too many questions.
I think love is kind of an umbrella term for a lot of different feelings, like the pain when someone you like to be around isn't around, the euphoria you feel when you're with that person, or the determination to do the right thing for that person. Normally I say love is goodwill toward someone, however little it is, but with "romantic love" I'd say it's when you prioritize that feeling toward someone above most other things.
Anyone can feel love. It might not always be healthy for a person or the object or their desires, and someone's view of reality might be skewed, but the feeling is there. People just aren't always the best at realizing when their feeling of love mutates into some other creature that's more self-centered like lust or jealousy because their actions might not change all that much (e.g. a jealous person will still want to be around you like they would when they loved you free of jealousy.)
I don't know why people seek love. Validation? And sometimes people don't seek love. I think love has a good reputation, it sells well, so people want it. It's also easy to mix up love with other feelings that might already be driving someone (like lust).