I've had a hard time maintaining friendships over time.
My best friend ever is my wife, we've been besties since 1990 (high school sweetheart).
The biggest problem I have run into with other friends is changing life priorities. I was close with a few guys in my teens, 20's, and into my 30's. Then it seemed like somehow a lot of them started living lifestyles that I didn't want to be part of. Quite a few of them got divorced after having affairs, became heavier drinkers, were back into the bar scene, etc. I was always focused on my wife and my kids, took my job seriously, etc.
When your kids are young it takes up a lot of time, so you don't have a ton of time to go out and make new friends. Plus, being busy at work, sometimes 60 hours a week, keeping my house up, etc., I don't have a lot of time.
I have become pretty close with my son's guitar teacher, the good one I talked about in the other thread. We are close in age, have the same values, similar viewpoints, and we have common interests. Outside of my wife and kids, who could never be replaced in my heart, Troy is probably my best friend. Luckily I have my wife, she's my bestest bud ever.