As members of athletic teams, we know and understand that the other people on the playing field with jerseys that match ours are our teammates.  The key thing to understand is that while you are practicing, playing, or training, the people that are with you are your teammates, not your friends.  Once the game, practice, or training session is over, they can become your friends. 

In my experience, many athletes have a very difficult time differentiating teammates from friends when faced with situations that call for such a division.  It's important that athletes learn to do so, as being a teammate and a friend are entirely different and must be treated accordingly.  To understand how these relationships differ, we must look at each one independently.

Teammate

Teammates are people that will challenge you to be better because they want to see the team succeed. They work with you, get up early with you, travel with you eat you because they are there to help you accomplish a goal.  They are willing to push you as an athlete and person to better yourself. 
They will encourage you to be improve your habits and discipline and may not be concerned with how you feel in the heat of competition and training.  They will tell and teach you to do something the right way if you are not doing so.  They will go into an athletic competition and give all they have for you.  They will not let you give up when things are tough.  They will tell you things you may not want to hear to for the teams best interest. Being a good teammate requires saying uncomfortable things and understanding that you may not always be liked. It’s not about making friends but earning trust.

Friend

Friends are the people you like to be around because you enjoy their company. You enjoy the way they make you feel and will offer comfort you when you are hurt.  They will treat you the way you want to be treated.  They will say things you may not always want to hear, simply because they care.  They will pick you up when you are down.  They will be there to hang out even if they may not know a thing about your sport, but you simply enjoy each other's company. Friends may or may not say things to make you feel uncomfortable but will often protect feelings first and foremost.

I've had a couple experiences with different teams who could not make the distinction between friends and teammates.  In these cases, the individuals were more worried if people liked them than they were about earning respect.  Let me give you an example. 

During a training session, a group of athletes noticed that their teammates were taking shortcuts (i.e. cutting reps, shorter range of motion, using lighter weights) and did not say a thing.  They continued to chat about what they were going to do after the session and weren’t focused on the details of improving.   These athletes were too worried what their teammates thought of them as people rather than as teammates.  Being friends is great, but when you are part of a team, being accountable to the group is has to be the number one priority.  As a teammate, you have to be willing to say and do the uncomfortable things, even if that means alienating yourself temporarily. If your team members truly care about the group, they will understand your perspective and realize that they can be better.

It’s important to understand that in competition, the members of the team are the ones that will have direct influence over the outcomes of the game. Coaches can implement tactics but it’s up to the team to execute collectively. Team accountability builds trust amongst team members. When you can trust each other in every situation, you will form a bond that becomes stronger and becomes more difficult to break.

Some athletes may think that if their teammates are on their case, that it will affect their friendship.  Things that happen in practice, game, or in training remain there.  Do not base your friendships with teammates on what you do in practices and games.  This can lead to problems if you have a problem with one of your teammates who is also a friend.   

Your true teammates are the ones who will push you to achieve things you never thought possible.  Your friends will be there for you when the game is over.