Composition is an important part of photography. The right lighting and all the bells and whistles in the world will not make up for poor composition. It’s also important to imagine a composition if it were to be cropped. A “bad photo” is almost always directly tied to poor composition.
The most important thing to ask yourself when cropping is “what is the subject of this photo?” If the subject in your photo is your friend, then you should crop out the drunk jackass spilling beer all over himself. If you are submitting the photo to the police to show the man was drunk before driving his vehicle into your sweet red bicycle, then you would crop out your friend.