I think it's different for everyone, but to me, when I did still feel it, it's a feeling of total adoration, admiration and dedication to someone/something. Everything about them/it is great, you can't find fault whatsoever. It's unconditional, nothing about them/it upsets you, every aspect/attribute of them/it makes you happy and makes you smile, and when you are separated from them/it for even a short while, there is an intense longing for the person/thing. It's a total feeling too. There is no room for any doubt, any negative feeling/aspect, it's perfect. When you feel it for someone for example, there is nothing about them you want to change. To you, the person is just absolutely perfect, totally flawless.

I guess it's kinda silly and stuff in a way. It makes you quite vulnerable, and somewhat blind for flaws everyone else can see in the person/thing.