Customers lie through their teeth, they'll rarely tell you the real reason why they dumped you. After a while you can guess who will mess you around, become a bad payer, do a runner and move out, etc, etc.
I never dump customers, always replace them. Only ever inform customers I'm stopping if they are all in close proximity or I clean a relative or friends house just to clarify there is no misunderstanding that I'm giving up window cleaning in other areas.. Slowly replacing town houses for larger, rural, country houses and manors, etc hence why they get replaced.