Firstly you can be surrounded by people and yet feel totally alone. Sometimes in a room of mothers I do feel they're all on the other side of the hedge from me.
Secondly there is an overwhelming sense that many avenues are now blocked off. I know that you can carry on as before even with a child, there are people that put a baby in a rucksack and climb machu picchu etc etc. However, these people are in the minority. Similarly, I have total respect for mothers who because of their job or hobby or ambition are able to go off and do something for themselves and leave their children for a period of time but I couldn't do it. (I wish I could!). Even once you're married it's difficult to say 'i'm going to do what I want or need to do' if it would be detrimental to the needs or wants of your partner but once you have a child this is magnified.
Thirdly you can feel so often that you have made a wrong turn in your day to day child-rearing or that you are going in circles.
And another thing with mazes is that you work so hard to find the middle and then if you get there ...so what? You feel great for a few minutes but then you just have to try and find your way back out again.