A team is built on the trust a manager instills in his/her team members. By allowing a team member to fail in a safe environment—FAIL Oh God the F word. Let me explain this further. For example, a fresher joined my team. We need to communicate with foreign clients on a daily basis (the emails contain a lot of info, and some of it is fairly complicated). At first, we sat together at her work stations and drafted the emails. Two weeks later, she was drafting the emails herself and mailing them to me for a look-over and corrections. When an email would come, I'd ask her questions: Have you considered this? What impact would this have on the deliverable? What are the implications of your deadline on the stakeholders? Have you OKed this with them before committing them to this deadline? After corrections, she'd mail it to the client.
In other words, she was allowed to make mistakes without impacting the client or fearing ridicule. FAIL in a safe environment. Now she can work independently, although she does make occasional mistakes; it isn't the end of the world.
Managers need to create a safe environment where a person is allowed to learn and grow, where managers don't take credit for the successes but share blame for the failures. By instilling confidence to make decisions and not micromanaging the team—of course, initially, the manager would need to closely monitor the team members and gradually back away as the team member gets better at the job. Most managers (and I'm included in this list) have trouble maintaining a balance between delegation and a telling approach.
While this approach can create a cohesive team, there are a whole lot of other factors that cause teams to break up that aren't in the manager's control. Examples I've dealt with:
1. Spouse relocating to another city/marriage
2. Designation and pay hike from rival organizations
3. Desire to relocate to the home city