Love means "willing the good of another". Simple as that. Not for my sake, but for your's and your's alone.

I can love a stranger on the street -- for example by moving aside, giving a coin, or smiling -- for his or her sake. I can love a teenager -- for example by correcting, hugging or advising -- despite eye-rolling and sighs. If I expect something in return, then it's no longer love.   

One desires good things for oneself; "love thy neighbor" simply means desiring that same goodness for the other person...even if it means less-goodness for oneself.

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My mom and I attended Mass at St. Patrick's Cathedral in NYC last month. During the sermon, the priest provided this definition of love: "willing the good of another". For some reason, it really clarified the concept for me. I thought I understood "love thy neighbor as thyself"; but understanding the concept of an idea and "attaining an idea" are completely different. Thanks, Priest!