Awwww. Someone appreciates you, Receiver. They think you're a damn fine human being. A gesture of appreciation as appropriate between friends as between lovers,
You're lovely simply indicates that the Bearer genuinely appreciates the Receiver’s good qualities, be that generosity, kindness, loyalty or simply good company.
Thank you thanks the Bearer,
Squee displays enthusiasm, while
Likewise or
You rock! respond in kind, and a smug
Yes,
About time or
My work here is done indicates the Bearer’s appreciation is considered no less than the Receiver’s due. The ends of romance are served with
You're beautiful,
I like you or
I love you. Receivers wishing to disclaim the compliment can do so cheekily with
Who, me? or mock-seriously with
I think not or
Unworthy, and those wishing the Bearer to cease and desist can indicate the fact with
Make it stop,
Creep or even
Go and die in a hole.