Bulbasaur reacts differently to that -- it leans its head away from the touch, growling softly.

"You don't like that, Bulbasaur?" Sam apologetically smiles at them both, then nods toward the bulb. "Try the bulb, Bulbasaur likes that." It was one of the first spots he tried when he first got it. "Just be gentle, and stay away from the very top and the back." At the top of the bulb is a small opening, and the back is where the vines seem to come out -- so it's understandable that those spots would be sensitive.