the reason (at least what i think) for me getting bored in single player is that i want feedback, like if i make something interesting or cool i want someone to give me their opinion on it and mabey help me make it better, i also enjoy it more just because if an interesting conversation starts up you can have alot of fun (not that you couldn't have fun just building) while still building your ship
Meanwhile, this is exactly the reason I prefer to do my building in SP mode...
I used to prefer building in MP, but after a while too many people knew me and so being on a busy server became a cascade of questions, requests for assistance, and requests to "come look at this" and "can i bring this by to show." Which is cool, but I quickly realized that I was spending 4 hours online and getting about 30 minutes worth of building done. Now I build in SP, I build much faster and with more focus, I do my Alpha testing there, and then I upload my designs to MP for Beta testing & feedback.
For newer players or players exploring new systems or build styles though, working on a MP server is invaluable. That advice and feedback saves you a lot of time.