For starters:
You spend almost every waking moment thinking about the person.
You put their needs before your own.
You always try to do things to make them happy and want to be a better person for them.
You can look them straight in the eye and tell them anything.
You light up when you see them
You smile more than frown
You don't look for faults or things to annoy you and accept them for who they are
You realize, despite whatever ails you at the time, that you are lucky to have that person in your life.
You can't imagine life without them

I should know. I am madly in love.

Love isn't any one thing, and it can't be strictly defined. There are many forms of love, and people see them differently. Love also isn't infinite, or simple, and nor is it alone enough to sustain a relationship.

For me, love is knowing that when my husband walks through the door, even after 6 years, I still get butterflies. Love is knowing that I would gladly die to keep him safe from harm. It is putting his needs before my own without thought of reward or return. It it knowing that he is my best friend, my lover, and the other half of my soul because I feel it every minute of every single day. When he's gone there is a hole inside me, because a piece of myself is missing. I could spend every minute of every day of the rest of my life beside him, and it would never be enough.

For me, love is all of these things and more, and yet they still fail to accurately and completely define it.

I can say that about all of them, that's the problem. That's why I keep mistaking the fear/gratitude/etc for Love instead of what they actually are - it seems oh-so-perfect and Right when I'm in the middle of it - then the cold reality of the aftermath analysis makes me aware of what was really going on. Hence, the reason I can't recognize the real thing.

Are some people born without the ability to recognize it?