first, this is not a complete sentence, right? second, well let me give it a guess anyway, i'd say "those things that are built beautifully usually are because people who built them enjoyed doing so" God, i don't think this is right lol~

There is no way to fully answer your question since the sentence is not complete. "... of building do WHAT??". However, I suspect that you are wondering about the term: "built for the joy of building" - anything that is "done for the joy of something" means that the primary purpose of the act was to satisfy the individual's or company's eagerness to do the task (in this case building) as opposed to the more practical or natural purposes of the task. So "built for the joy of building" means that someone built something because they like building things and it made them happy. Their happiness derived from building, was much more important to them than whatever the normal use of the item would be. (I built this car for the joy of building it. However, it only gets 1 kilometer per liter and is not very practical, so I do not drive it much.)