That term is thrown around so much, and can have a variety of meanings to each individual. You can love friends, family, pets, passions, etc. The one thing I can say all these thing have in common is there is an emotional component to it.
I can tell you what it means to me in regards to marriage. Today is my 12th anniversary with the woman I decided to spend the rest of my life with. We started this journey long before the marriage came into conversation. Total years teeter around 19/20 depending on who you ask.
We spent the first couple of years trying to become familiar with each others personalities, and everything that comes along with. There are so many layers to a person, and just like an onion you continue to peel away till you get to the core. Sometimes cutting too quickly can lead to tears. If you are like my wife, you put on sunglasses to help prevent the potential impact of those chemical irritants. :)
Depending on the state, there could be some damaged pieces, and you have to decide how to proceed. Do you just throw it away and look for one in better condition? Or do you cut the damaged area out, with hopes of preserving the remaining good?
I can say this is how I have viewed our marriage in it's various stages. Everyone have damaged pieces to them, and the approach in how you remove those to expose freshness will dictate the longevity. We all know it may not work every time, but you have to willing to take those chemical irritant induced tears that come along with the new layer.
Understand the percentage of ill intent for a loved one is rare. Not many people would waste their time/energy just going around hurting people for the hell of it. Usually we carry the damaged pieces with us into these new relationships hoping they will just never expose themselves.
Life lesson. Expose it early and often. That person you have chosen to be your life partner is there with no stipulations. Now the opportunity to experience those new layers will be a healthy partnership. As the great James Baldwin says "Not everything that is faced can be changed, but nothing can be changed until it is faced." It took us almost 20 years to figure this out together.
I'll leave you with this. It's never too late to do the right thing. This can mean whatever applies to your life, and makes you the best version of yourself everyday. Until this happens, you can't be the best version for anyone else.
Marriage is a career, not a job. Built for longevity with vested interest in growth!