I think many people see 'the one' as some miraculous being that ticks every single box from tall dark and handsome, all the way to caring, listener and idolises you, with several hundred expectations in between.
Obviously this is totally unrealistic but were told that 'the one' will come. When people hit a road block in their relationships, due to whatever problem that's occured, they're quick to assume that the grass is greener, only to realise that it's certainly not the case...afterwards.
Everybody has faults and characteristics that we might not favour, it's about dealing with them amicably and learning to move forward together. Nobody is perfect, the grass is greener relationship only comes with a new set of faults and flaws.
In my opinion, you've found your person when you find somebody who is willing to hear, own their 'stufff', grow with you and try new ways. So many problems could be solved if we were all willing to hear, without getting defensive, which only causes arguments.