Having spent almost nine months eating, breathing, sleeping, and doing everything else with one person, it's hard to fathom what summer vacation will be like without your roommate. Initially, the space is good for both of you, but by July, you're ready to do everything together again.
After graduation, there comes a point when the three month eternity that was summer vacation will turn into an unknown amount of time until you see the person you've shared every waking moment of college with again. That feeling in July's past of wanting to get back to school is only going to get more and more unbearable as the realization that you're not going back to your old ways becomes more apparent.
As much as you both needed space here and there over the past four years, you'd both give anything to have just one more night together living in the same room, annoying each other one more time by keeping the light on or watching Netflix without headphones or disrupting any of each other's pet peeves because you didn't realize then how much you'd miss them now.