1. You like their friends: If you can't get along with their friends, or the friends are always butting into your relationship, it will evetually have a negative effect.
2. The follow through: If they don't then they may not really be into it or into you.
3. The show you they care
4. They want you around: Kind of self explanitory, if you don't to be around that person, why would you have a lasting relationship.