Is jealousy a milestone too?

We bought a walker for Ashu this weekend and she is happy. Yesterday she was jumping and today she walks the entire house. It won't be a big deal now when she actually walks on her own I suppose! Everything is so gradual that there is not one moment you remember. She was trying to roll over for a long time and the progress was inch by inch. Then she crawled, sat and stood. It's one-step a day and probably I ve to fast forward like they do the flower blooming to actually see her do stuff. The other day I picked up my friends 4-month-old baby girl and was kissing her. Ashu started screaming all of a sudden and I was shocked. I have one mean little thing in my hands. This development was nt gradual though. It came out of the blue. Girls!