Good or Bad: Dating someone "out of your league."

I'm relatively new to the world of Western (American) dating, and something about it really confuses me:

One of my (male) friends is going out w/someone who (many of his friends think) is "out of his league." It almost sounds as if they're making fun of him when they say such a thing. From what I understand of "leagues" it's all based on how attractive a person is. If this is the case, is it not better to go out with someone of as high a league as possible, or am I missing something here?

What are the advantages/disadvantages of going out w/someone "out of your league?" Are his friends just making fun of him due to jealousy, or is there some legitimate reason why he should not be dating someone that's more attractive than he is?

Thank you in advance for your advice.
Good or Bad: Dating someone "out of your league."
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