Here is more text:
"My lifelong best friend, larry, who has always been my lighthouse. He cheers me on or helps me focus, and he never lets me take myself too seriously."
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There is no idiom at work here. People who "take themselves too seriously" are prone to think that the things that they do are very important, that the things that happen to them are significant, and generally act as if people around them should recognize this importance and significance and treat them accordingly. Therefore, people who take themselves too seriously can be unpleasant to be around, and the speaker is saying that his friend helps keep him from being that way.