I am going the solipsistic route, because it's the relevant one in getting to the bottom of what these fundamental things actually mean. The point is, love is something that can only exist inside our individual minds (what is a mind, even?). Scientifically, we bare know what it is (something to do with dopamine), much less are we able to measure it. When you love your spouse, you take on faith that the feelings you have inside are what love really feels like. You tell the little voice in the back of your head that asks "Is this really it? Ohh, the model is that ad is really good looking, perhaps if..." to shut up. But you don't go to the love doctor and have your heartbeat or whatever measured to empirically confirm that your feelings are truly love, and not merely affection, an infatuation, or lust.

And that's without getting into what "actually knowing" someone really means, and how you evaluate your spouse's claim that she loves you. At least you can feel your own feelings.

I'm not saying that these feelings aren't true, I'm sure they are. I'm contesting your assertion that you take nothing on faith.