[tome]

The way I interpret is as follows.

Forcing yourself to do something (like smile) when you don't want to is like erecting a tall building. The more you work on keeping the structure of the building steady the more the more you are required to consciously and unconsciously work on solidifying the foundations and the structural integrity of the building.

(This is just one outcome of course. Another is that you keep forcing it and it doesn't help you get the foundations in order.)