Being on the path of self awareness and growth, I was taught that being alone was essential for my development. I don't think its true that a good measure of mental health is the ability to be alone. Sure, there is something to be said for being comfortable in your own skin, and we often use relationship to distract us from uncomfortable truths about ourselves. But in the end, what makes the biggest impact on mental and emotional health is the ability to self reflect: To not place blame. To own our experiences. To express our needs and desires. To know our limits. To be with uncomfortable feelings. To admit when we are wrong. To listen to others.
All this can be done in the context of a relationship. In fact, I'd say these skills can ONLY be mastered in relationship. It is relationship that creates the atmosphere to practice these things.
Our relationship with our friends, family, and partners are the swiftest path to growth. It is only in relationship that we can see ourselves- our beliefs and truths differentiated from the other. We see our uniqueness when contrasted with someone else. AND we see our sameness and connection as we support of friends in their joy and their pain. One way to feel connected to life force is in seeing our own vulnerability and power reflected back through another. Also, without relationship there is no empathy. When we isolate ourselves, even in a noble pursuit like a meditation retreat, we cut ourselves off from experiences that create empathy. Roger Ebert once called movies "empathy machines". He recognized that it is as we experience life through the eyes of another that we really open our hearts.