As pretty much everyone has already said, the harsh truth is that most of your friends, aside from your truly close friends, will walk different paths, choose different routes, and basically drift in their own direction. For me another difficult things to adjust to is that even if you continue to play collegiate sports, your old teammates will either be done playing sports all together or at a different college on a different team. When you have some teammates you've been playing sports with for 10+ years (sometimes even longer) it is extremely sad when you all go separate ways and you won't put that uniform back on and fight together any longer. Although, to lighten the mood a bit, aside from working/schoolwork I've had quite a bit more free-time since I graduated a couple years back and you have plenty of time to focus on your passions and goals, self improvement as well.
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Don't cry because it's over, smile because it happened.
I am a believer that there will never be another game or time period that can bring as much joy to those of us who spent many long nights on mw2 on xbox live living our best lives.