I my year of aveilus for my mother I had 2 chavrusos. One was Rav Moshe's son, Rav Reuvein and another boy named Grossman. That year both my chavrusos got married. When Grossman got married I asked if I may attend his wedding. Rav Moshe ruled that I may go as a waiter. A few months later I asked if I may participate in Rav Reuvein's wedding and Rav Moshe ruled that I may not.
I asked him what the difference is. He explained that Grossman needed you at his wedding because there weren't going to be so many people to add to his simcha, so I was lenient and allowed you to go. But my son's wedding will be attended by hundreds of people including Rabbonim and Roshei Yeshiva so if one of his friends doesn't attend he won't feel less simcha, therefore I was stringent.
Note: From here we see that we should never decide a halacha based on a story for if the circumstances are different the halacha may be different.
