Why are bar mitzahs good to attendend? What good can you gain?

A Bar Mitzvah is a critical milestone in the life of a Jewish male. It's when he becomes a "Son of the Covenant", and an adult in the eyes of the congregation, ready to take on the responsiblities of a Jewish adult. This is symbolized by his sitting on the bema and reading from the Torah in Hebrew, reciting the accompanying prayers, and giving a speech. In short, it's a very important, very sacred day in his life. If he's invited you to be there, it means that he considers you a close friend, and someone he wants to share the experience with.

If you're Jewish, being there symbolizes the importance of friendships within the Jewish community, and sharing milestone events in one another's lives. If you're not Jewish, being there will provide you with insight into your friend's religion, showing that there are many ways to believe in the Lord.

While sitting there, you will also be learning deep philosphical lessons, gained from the readings from the Biblical texts, and a better understanding of what it means to be part of a religious community. There's a continuity that extends back at least 5,000 years, to the original Coventant between Abraham and G-d, and if you listen and watch carefully, you'll gain a understanding of that relationship and why it has helped the Jewish people to survive all those centuries, despite persecution and affliction.

In a nutshell, that's the importance of a Bar Mitzvah, and why it's good to attend.

As for any reception or party held after the service, that's separate, but it's still good to celebrate with your friend and his family! There's a lot of sadness in this world, as your mother no doubt understands, and when there's a reason to celebrate, you should consider it a blessing.