Attributes Needed to Make a Frienship Last/Grow

Hey all! Thanks to everyone who is following this! This post comes from a conversation I had with a good friend who wanted my thoughts on a list of attributes needed in order to make a friendship grow and remain strong. This is what my fairly simplistic brain achieved in vomiting forth. :-) Enjoy! Sorry about this being kinda long.
What attributes must exist to strengthen and grow a friendship. [note: this is not an exhaustive list].
1. There has to be a common concern.
Many people are in "one-sided" friendships--which really aren't friendships at all. If there is no common concern for each other, then there cannot be a friendship.
2. There has to be a common appreciation.
If both people don't appreciate (have high regard for) one another, then it is very hard to have a friendship Now, that regard may be higher or lower at times based upon circumstances, but it should still be there.
3. There has to be common interests.
Two people who have nothing in common rarely have good, close friendships. Some people will have more in common than others, but some people just have to dig deeper to find something in common with each other.
4. There has to be common goals.
This is especially true in the view of making a friendship last and strengthen. It is like strands of a rope. As long as they are pulled in unison and in the same direction, they remain together. But if you begin to pull them separately and in different directions, then they unravel and once unraveled, are very hard to make "one" again.
5. There has to be reality.
What I mean by reality is this. Strong friendships exist in a real world. They are not things that exist in a utopia. In a real world friendship, there are going to be problems. To be naive and think that you can have a friendship (even a strong one) that won't have ups and downs is to try and have a friendship outside of reality--it doesn't work, and will not last. In a realistic friendship (esp. of 2 Christians), there are allowances and expectations of failure, mistakes, and sin. A friendship of one person almost "worshipping" the other person will always eventually be destroyed because that person who is worshiped will eventually make a mistake "too big" for the other person to handle.
6. There must be a common desire to build and strengthen the relationship.
A friendship can last for a decent amount of time in "stasis", but not indefinitely. At the same time, a friendship cannot be improved by one person alone without participation/desire by the other side to improve it. Now, one person can take the lead and be an example and turn the other person from not caring about improving the relationship into desiring and working to improve it based upon the improvement they perceive in the other person. (let me know if that doesn't make sense). While at the same time, a person who has flatly decided to not "work on" the friendship and just lets it go--and is unwilling to change from that course--will destroy the friendship. This sadly is what happens to many marriages where the two people are married but no longer friends (or at least not good, strong friends). One or both parties has decided to take the friendship for granted instead of striving to continually improve it. Until there comes a time when both parties will work on the friendship, there can be no true improvement in the relationship--only in the individuals.

~Aaron