Why does relationship burnout occur? is there any way to avoid it?
There is no way to avoid it
Your example is a great start. Add in the opportunity cost and you start to get an overall model. Keep in mind that everyone is different so it isn't one answer. But ever heard of a guy break up and say the reason was because she was too clingy? Well guess what that means? Diminishing marginal utility tanked. Now I can only eat so many slices of pizza for dinner today then that's it for pizza. But guess what? I can pull the pizza out of the fridge the next day and eat a slice for breakfast. Maybe even sooner, maybe a late night snack. So yeah, in a relationship, I might need some time away, maybe a boys night out even. Then start again. Another thing is what is on the pizza. Say we bought 2 pizzas with slightly different toppings. I might have a slice of one, and utility drops. But I then might have a bit more pizza than usual just to try out the other topping. And in a relationship you can be something like a pizza, but just change the toppings every once in awhile to switch things up.
The relationship is not that way, because it is priceless. Your marginal utility will be diminishing, but very small at a time due to your age. It's the same as diamond, but not water in economics.