When running with people, you are able to push or be pushed to a higher level of running, when running with Jordan and Ashley, I am at a place where I can push them or encourage them to improve their time and distance. I am put in a place of responsibility, where I can leave them in the dust, or stay with them, and help them do their best. In both cases, they were able to do their best. Not because I was cheering them on, shouting or whatnot, but because I was simply... there.
Let's think of a new scenario. When you put an veteran runner and a novice and have them run together, the novice will improve at a rapid rate because running with a veteran insures that the novice will gain experience at a faster rate than if they were running alone. The reason that they will gain experience at a faster rate is because of the push that the experienced runner gives them. During the run, there are many opportunities for the veteran to encourage and advise the other runner, as well as pushing the pace ever so slightly, using synergy to speed the time, and the adrenaline to push them further.
Having someone there helps, whether it be running or anything else. Being by yourself, you can discipline yourself and improve, but having someone teaching you, guiding you, encouraging you... you can go a lot further. When you have a math problem that you don't know how to solve, you would look for someone that understands the problem and can explain it to your level. If you want to be a little more outgoing and get to know people, you hang out with people that are outgoing, and little by little, they rub off onto you.
Hang out with the fast runners, and you will improve your speed. Hang out with the distance runners, and you'll improve your endurance. Hang out with the good crowd, you become even better.
Who do you run with?