Pendulum swing and balance

It's so odd. Suddenly I can get deeply motivated by science again - and not just half-heartedly. For a while there I felt a loss in fulfillment and meaning from my research. The only motivation to get it done was that it was my job. I wasn't even sure if I wanted it to be my passion again.

I'm very relieved to have the pendulum swing back in this more satisfying direction - but I'm also wondering why the heck it keeps going back and forth. Does this happen to all scientists (but maybe to a lesser degree)?

What is additionally strange (but I'm not complaining) is my shift in work/life balance attitude. Where I previously envied my friend who could take a year off for maternity leave, and then return to work without missing a beat (she's an ER nurse) - now I'm pretty sure that I would miss doing my research, and that I'd want to get back sooner (much sooner) than a year. In fact, I could even see myself enjoying my work enough to pull those strange hours that I used to think were crazy (like working again at night once the kids are in bed) - as long as I had quality time with the hubby and family too. I used to think that type of lifestyle was unbalanced. Now it seems pretty nice - but only with the major caveat that I really loved what I was doing.