When I was a kid in the 80s I always used to look at the pictures in the colour ad brochures that came with each set. I used to be obsessed with them and dream of one day creating my own. I wanted each and every model and make a lego town of my own, but bigger and better than all the pictures put together.
I still have that dream.
I think on the whole though I like Lego for the same reason I like airfix, DIY, Gardening, Astronomy etc. The simple reason that you can see the fruit of your labour and think "I did that, and it's awesome!" With work I'm constantly looking at the next thing I have to do, I never have that sense of something being finished, because the moment it is, there are a million and one other things that need doing.
I think it's also because when you're playing with Lego, you experience what Mihály Csíkszentmihályi described as a state of Flow. Flow is the mental state of operation in which the person is fully immersed in what he or she is doing by a feeling of energized focus, full involvement, and success in the process of the activity. So much so that they can loose almost all consciousness of anything else that is going on around them and time seems to pass extremely quickly.
As others have mentioned, Lego Town offers endless possibilities and it can always be improved, expanded, redeveloped. For me, that means endless times of feeling happy about what I've worked on, seeing what I've acheived, and being totally absorbed in the process.
I'm planning on starting to build my Lego town shortly. Just need some money and permission from my wife!
Maybe one day.
Mark