Stewie and Brian were trapped in a bank vault...they went through many emotions as other shows have done, when people are put in a tight (literally) spot and they 'have it out' with their feelings and such...but Brian admitted he had a gun in his safe deposit box, if he wanted to commit suicide. Stewie is surprised, and just asks non-serious questions first, but then asks Brian why..Brian said he felt like his life didn't have meaning.
Stewie reflects, and first said that it was a selfish act...well at that, I was ready to stop watching, because in my life when I blurted to the one closest to me that I was so despondent that I felt suicidal, and that person said it was the most selfish thing he had ever heard..
but then Stewie went on and said 'but Brian, you give MY life purpose' or something similar to that.
I'm probably not relaying this very well, but it was just really touching, and made me tear up, because many of us have been at that point, and maybe all it takes to keep you from following that train of thought, is that you give someone else meaning in their life, even if you can't see that, and think your life is worthless.