Spend More Time With Their Friends
It's healthy for couples to spend time away from each other with their friends - in fact it can even help strengthen a relationship. However, if you are spending more time with your friends because you don't want to spend time with your partner then there is an issue. You should want to create memories that will carry you through a lifetime. In a nutshell, you should enjoy your partner's company.
When your friends plan a couples night and you opt to go solo, then it's safe to say that you're in a relationship rut. If you do not want to introduce your friends to your partner or if you would rather hang out with them instead of having a romantic night out with your loved one - then you're in a relationship rut.