We've been married since 1956. Dated for 2-1/2 years before that. In my opinion, it gets easier as the years pass. The rough edges come at the beginning and once you've smoothed those over, and a few years pass, there's usually no more surprises. You know each other's strengths and weaknesses, as well as each person's sensitive points. It helps if you are truly in love with your mate and it's reciprocal.
Kids come along, you weather the storms together and as they grow and pressures ease, much of your time is spent recalling wonderful memories and also some of the sad events that you've shared along the way. Except for health problems that creep up on all of us as we age, these later years can be "golden". If you are fortunate enough to be surrounded by a loving multi-generational family, that's the icing on the cake.
Heading towards our 62nd anniversary in a couple of months and looking back, I wouldn't have had it any other way.
My disclaimer: This is only our experience. For many other well-meaning folks, and in some instances, due to situations beyond their control, life has taken them down a totally different path.