How come a lot of people change their behavior after marriage as if their waiting to exhale??

I am going through this problem now. I feel as if it is always at least one spouse that has changed, not all the time. Although, change in behavior does occur quite often. For example, been married only 6mths. and already there is a change in my husband. Use to be fun, now he complains his way out of sthg., I realize he doesnt comprehend a lot of things and in denial about it, do aggrevating silly things now to get attention(ex.stare &standing in front of me naked) , drive decreased to twice a mth. sometimes not even. I was not expecting to see changes this drastic so soon after marriage.