I had a similar experience with my DD, now 5-1/2, who spent 2-1/2 weeks in the NICU. I had less time off from work after her birth, which I think played some role. But for me, I think the major factor was that I was afraid to get too attached to her because she was in the NICU. Even though my rational mind knew she was almost certain to come home with us and be fine (she was only in for feed and grow, never took oxygen or had As or Bs), I was holding back, keeping myself from getting too invested. That dissipated after she had been home for a few months and stopped being so unbelievably tiny and fragile looking.
DS (8-1/2), DD (5-1/2), #3 expected 1/5/09