(Reason Number 4.)
A big part of keeping a long lasting friendship is being supportive of the other person. This means being there when they have a problem, valuing their feelings and opinions, and helping them if needed, no matter what. It also means that you stand by their side, even if you may not agree with what they are doing. It also means being honest as well when you don't agree with something, but also to let them know that you will be there for them if the choice they make ends up not being the right one.
Being a supportive friend means that the friendship is equal and neither one of you both have an equal share of the responsibilities. It also means that you are there when you need them, whether it is taking care of their kids if they have to go out of town in an emergency, or coming over to check on them when they're sick. It also means that you contact them when you haven't heard from them on a regular basis to make sure everything is all right.
Being supportive goes both ways. If your friend isn't by your side in times of need, or is always making excuses about it, are they truly your friend? This isn't about using them; it is about knowing they are going to give you a hand. It can mean coming over helping build that garage when they have time, or even giving you real advice when it comes to dealing with problems in your life, and standing by you even if they don't agree with your decisions.
Being Supportive to your Best Friend
What makes a good friend?
Lessons learned for "being supportive" in your relationship