If they make you feel good, keep them close.
If they motivate you to do great things, keep them close.
If they are not afraid to tell you the truth about yourself, keep them close.
If they love you beyond the small fights and quarrels, keep them close.
If they still lookout for your good even when you wrong them, keep them close.
If they say sorry even when they are not at fault, but want to restore the relationship, keep them close.
If they are honest, keep them close.
If they ever at any point of their life placed you as a topmost priority, keep them close.
Keep them close and never let go because people who are like this are not always available. You may let them go and then realize they are irreplaceable.