It is interesting how time and events can change your perception on things.
The major one, as I have pondered before is how ones perception of time changes. Interestingly, it is often time that changes your perception of time. One way to see this is thinking back to how long days and years took when you were younger, seemingly stretching on for ages, compared to an older perception when it doesn't "feel" as long. Note that time itself isn't speeding up around you, it is your perception that has changed.
Another example I noticed earlier today was when I was holding my son, and he was looking out over my arm to his mother. I suspect he was the first male to stare about 20cm below her face that she didn't complain about... So events also change perceptions.
I would say the biggest event of all that can change your perception on something is experience. It is easy to have an opinion on something never experienced, but once you have gone experienced it, your perception on it can, and often will, change. Knowing this (or at least expecting it), I tried to enter parenthood with as few perceptions or expectations on it as possible. So my experiences will be painted onto a blank, untainted canvas, if you will. I think it has helped in me adapting to the massive change so quickly and, like Liz pointed out, it feels natural and as though he's always been here.
