I'm thinking of opening my neighbor's mailbox.
Why do you think it is okay to open a person's mailbox? It is against the law.
I think it will be fun
You want to aquire some sort of fun at your neighbor's expense and mailbox. You know you can be arrested for tampering with the mail that way. Refrain from doing anything of the sort, and life will be easier for you in the future.
Screw my neighbor, what if I open his door and his bed as well. This kind of satisfaction is how a human brain actually works.
Unless you are giving him a missent letter opening someone else's mail is a federal crime. It comes with a $300k fine or 5 years in prison