Relationship Ruts

Getting a long term relationship out of a rut and tips ideas? Why do ruts even happen? Why can we all just stay in the honeymoon phase forever? I’ve been with my bf six years and we are in a rut. We are also in a rut personally, we’ve both let our weight get out of hand and just don’t feel good about ourselves so I do think the personal ruts are affecting the relationship as well. Please don’t tell me break up like couples don’t fall into ruts especially long term, it takes actual work to maintain a relationship and it’s easy to fall into a routine (which we have)