Whenever anyone asked me (either during my mission or since) what my favorite part about being a missionary was, I always said it was having a companion. This generally surprised people, but it's true.
I loved having someone there to talk to all the time. I loved having someone there to always try to help me and serve me, and I loved having someone there to always try to help and serve. I loved that we both struggled and rejoiced together. I loved having someone to hug every night before bed and to say goodnight to, like I used to do when I shared a room with my sister.
I had really great companions. I never got sick of having a companion. I never felt like I didn't get enough alone time. I never felt like I wanted to "get away" because I needed space*.
Now I'm off a mission and companionless and back to my old cantankerous self, but that was my favorite part of being a missionary (or at least, one of my favorite parts).
*With one notable exception one day, but I was switching medications and it was in the MTC, and things get a little touchy in the MTC because you're still trying to figure out what you're supposed to be doing.