Why is "I" the most important ingredient in Team?

In any situation where one depends on others to contribute their best, there's alway's the question, Can they perform? Managers and team leaders go to bed worrying about this. The manager is given responsibility for a project, he/she has to decide on the make up of their team, I am not going into how one might choose his/her team at this juncture, but decisions are made, team is in place and off we go! The manager should be confident in their ability, their subject matter knowledge, experience, style....etc? They must know their team capabilities on an individual basis. So the questions; Do I know them? Do I trust them? Am I issuing the right instructions? There's three I's for the leader, there are more(ask your own), but I am just making a point. The team members now asking, or should be asking themselves: I have to know my job?, I know people depending on me?, I have to execute in a timely manner?, I have to be reliable? There are a lot more questions(ask your own) but again just making the point. Understanding the "I" in team working is its strength, it's the most important starting position one(I) can have. Nobody knows one better than one's self. To quote William Ernest Henley, "I am the master of my fate, I am the captain of my soul" (From his poem, Invictus)