How to love someone 'too much'
It starts off as a crush, then infatuation, then it becomes an obsession. No one holds the word obsession in a good light, maybe some do, I don't know.
Loving someone "too much" is when they're on your head constantly, not in the 'Oh, they're cute' or 'I wish they were here' way. It's more of them plauging your mind with every single action you do, or what someone else does. You connect them with something that has no correlation whatsoever.
You depend on them for everything, when something goes wrong you expect them to fix it. You expect them to be there 24/7 and only they can fix your problem, no one else. There'll be crying and desperate wishing that they'll fix it, when someone comes in to help, you refuse it. You depend on them to live. If you can't have them, if they don't show you enough love, then you're better off dead.
There's also seeing no flaws in them at all, everyone has their flaws, and you can love them. You shouldn't glorify those flaws, you shouldn't worship them as a god who'd you do anything for.
It gets to a point where you need to know everything they do, at all times. You try to help them in tiny ways, then you'll start micromanaging them because you know what's best.
It's unhealthy and I'm not the best person to explain it. Let's hope someone else can.