At the risk of stating the obvious, God uses relationships to build people. This is clear from Jesus' ministry (time spent w/ his disciples), Acts 2:42-47 (the word "together" is used repeatedly to describe their fellowship), and all of scripture. God didn't need to make it so. How could have made us to be lone rangers, but he didn't. Ecclesiastes 4:12 explains "Though one may be overpowered, two can defend themselves. A cord of three strands is not quickly broken."
I recall Chuck Swindoll sharing a story about a Marine Corps buddy of his who found more sincere relationships in a bar than in the church (http://tatumweb.com/blog/2007/07/22/church-bar/#comment-67001), and frankly I can relate. When I came to faith in college through InterVarsity Christian Fellowship, I belonged to strong fellowship and it made all the difference. In the 12 years since then, the depth of my relationships have been on a steady decline, for a number of reasons (most of which are my fault).
I'm convinced that church structures must foster healthy relationships, between believers, and reaching out to non-believers. These relationships must be God-honoring and real. Why is it so important that church structures support relationships? For some people, relationships come natural - they don't need structure. For others, either relationships do not come natural or they're just choked out by business. We must strive to ensure that nobody "falls through the cracks", neither visitors, nor regular attenders. BTW, I believe that small groups for all are great way to do this.
As Mother Teresa said "The most terrible poverty is loneliness and the feeling of being unloved." God forbid people feel the way Rich Mullins did in his song Elijah "There's people been friendly, but they'd never be your friends. Sometimes this has bent me to the ground."
A desire to serve the living God is the reason that I must be a part of a church. But the times that I've experienced sincere love (and there have many) are what makes me want to belong to a church.