The internal warmth from the glow of saving that money instead of hiring a plumber lasted at least 3 days.
Another part of the equation is - don't be afraid to ask for or hire help. Some people take on some projects because they don't want to or don't know they can ask for help.
the ikea effect in action...
I don't think it applies in this case however. There is no explicit cognitive bias at play. There would be if he considered his job is as good or better than plumber's (although actually, unless he has a very particular toilet setup, doing a good job after meticulous research and preparation is not out of reach at all).
In any case, even doing an OK job which is definitely worse than a plumber's but holds (and will presumably hold) is still a win if you're short on money but can spare the time/motivation.
Then again, I used to work in a retail position where I assembled so many things with far inferior construction and instructions. All in a day's work.
OTOH when people kitbash or modify Ikea furniture, I think that extends a bit into DIY. But putting it together out if the box doesn't count IMO.
I think some people just have a mental block when it comes to this type of activity. Imagining how two parts will fit together in the physical world or perceiving an order of operations to assembly, even if laid out in an easily parsed instruction booklet, just causes their brain to disengage for some reason.