Tension and not being able to relax?

What does it mean when two people are hanging out or even living together for awhile, but constantly feel that tension (not anger or fear, but the one that doesn't let you relax and be yourself). They only exhale when they close the door from the other side (like going to work). I know it happens a lot, when people just start dating, but someone told me they still have it after 7 years of being together!!