They're the first person you think about when you wake up and the last person you think about at night. A day without some form of contact, either physical contact or through other means, feels like a day that's empty. Going to bed without them beside you feels wrong. At first whenever you think about them it's lust and infatuation - there's excitement, there's desire, there's longing. They're a source of passion and intensity. Later on, after a while together, things become more secure. There may be less lust, but there's more calm respect. There may be less sexual intensity (or not, good for you) but there's more comfort. The passion is still there, but they become your other half, and it's more muted and based in long term goals then in the initial whirlwind stage. Their needs become just as important as your own, if not more so. Them being happy makes you happy, and you would willingly do things you don't enjoy as much as them just to see them smile. You think as 'us', not 'me and them'. When you can't imagine life without them in it, and you know that you want them in your life for who they are rather than what they can give you, then that's true love.