I don't breed but I'm good friends with the breeder of my dogs and I know many more through showing.
It's a huge amount of work and very expensive to breed responsibly, not to mention emotionally draining even when everything goes well.
There are so many things to consider before you even get as far as a mating; showing/working/competing/some kind of independent evaluation of your bitch, health testing (which can be very extensive and expensive for some breeds), finding the stud dog who is the best possible match for your bitch taking into account conformation, temperament, health status, pedigrees and inbreeding coefficients. The best stud is rarely the most local so there's the costs (and time investment) of travelling to the stud, plus potentially accommodation, to take into account on top of the stud fee.
Even when everything has been done right things can still go wrong during whelping and breeding expenses aren't covered by standard pet insurance policies. A good breeder needs to recognise when things aren't progressing as they should, or when a bitch is struggling, and get suitable veterinary intervention. Ideally someone breeding their first litter would have an experienced mentor on hand to help them to help.
Raising puppies takes a lot of work, it's not as simple as just letting the bitch get on with it. There's a lot of socialisation to be done on top of all the washing, cleaning and feeding (once the puppies start weaning) in order to give the puppies the best possible start in life. This website gives you a good idea of all the things a breeder should be doing during the various stages of the puppies' development.
Finding good prospective owners who have really done their research and would make ideal homes is tough. It takes a thick skin to be able to tell people they aren't the right sort of home, or to let someone down if they seem great initially then concerns creep in during later conversations. It's not always possible to do home checks if puppies aren't staying locally and even if home checks are performed there's always an element of risk. It's always sad letting the puppies go, even when you're sure they're going off to excellent homes.
It's certainly not something to enter into lightly and responsible breeding is pretty much mutually exclusive with making any kind of profit.