My husband and I have been diving together for over 30 years and so far we are equally avid and well-matched. As we get older there may be health issues that could make it more difficult for one or the other of us to dive, so we will have to see how that works out. But I think that if one of us has to stop diving the other will also retire from the sport.
If one spouse is willing but less eager than the other - my advice is to be patient and give it time. We took a YMCA pool course to get trained and I hated it. The big problem was that I couldn't see anything because I couldn't wear my contacts since we were constantly ditching and donning and taking off our masks. We also took the class during January in Chicago - so I was freezing the whole time. But after I had a few dives in the warm waters of Florida and I could see; I started to feel comfortable and began to really like it a lot!
I was also less comfortable in the water than my husband. He was on the swim team in high school but I had to take an "adult learn to swim" class before learning to dive. The first class was "introduction to water" and we all lined up at the shallow end of the pool and practiced putting our faces into the water!

It took some time for me to feel as comfortable as him but he was patient and I stuck with it and soon I began to enjoy swimming and diving.
One of the reasons we started diving was because it gave us a chance to spend time together doing something special. We are just warm-water OW vacation divers. We used to do 2 trips a year when we were younger but now we only do 1 trip per year. But we love visiting beautiful, exotic, warm places on the ocean, and diving.
So far we have done almost 600 dives and we have done every one of them as buddies. If one of us has ear problems or isn't feeling well then the other one sits out too - because we want to spend time together - we work long hours and this is our chance to relax and enjoy each other's company.
Sticking to the "both or neither" promise has been difficult at times, like on a liveaboard when there is plenty of great diving available and not much else to do. And I remember a trip to Bonaire when my husband had a head cold and ear problems and couldn't dive most of the week. I spoke with the dive operator and asked if we could "get a rain check on our pre-paid shore/boat dives so that we could come back later" and he told me "Your husband can have one but not you, because you are still healthy and able to dive!" He didn't understand when I said that I didn't want to dive without my buddy.
There have been many other activities and hobbies that one of us liked but the other didn't - but I am glad that we've agreed about diving - it has given us a lot of good times and great memories. And I am happy that I stuck with because I would have missed many wonderful experiences and the opportunity to share them with my husband.