Each interaction you have with another person, whether directly or indirectly, is a relationship and gives you the opportunity to learn something about yourself and make a positive change. Each one activates emotions within you. Being aware of what these emotions are and analyzing what within you is reacting to the other person or their actions is where the opportunity for your growth comes in.
The relationships in your life are the only way you can tell whether you're making progress in your spiritual growth. If you are always in a state of reaction, letting life in the form of other people control your life, you not only have more work to do, you know right where to start!