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The structure of most stories is that the main character is led to make some fundamental choice of values. Secondary characters exists to be the subject of those values (the love interest), the promoter of those values (the wizard), the supporters of the values (the companions), a temptation from those values (the siren), or the opponent of those values (the villain). 

No secondary character carries the same moral weight as the main character. No other character is placed in such morally complex situations as the main character. The result is that while the secondary characters can be and often are somewhat anarchic and fun, the main character is either morally earnest, or morally compromised. 

It is not surprising, therefore, the secondary characters often seem like they would be more fun to hang out with. The main character is often a bit of a bore or a bit of a sleeze.