Actually, only some of those scenes you mentioned made me tear up. The rest of them not at all. That's not to say I didn't cry in the movie, I did. It's just that the part that felt like a suckerpunch came right after 'Andys playtime' was about to end.
Remember that moment when the little girl looks into the box all excited and shouts out, 'my cowboy?'. Andy looks in and suddenly finds Woody, lifts him up and confused and puzzled, zones out the little girl saying 'There's a snake in my boot', tugs the little cowboy's string and hears him say the same? Excited, she reaches for Woody but then...
Andy suddenly pulls back, horrified.
Those next 3 seconds depicted one of THE most emotionally powerful scenes I've EVER seen in film. The disappointed look on the girl's face, the entire transition of emotions shifting with Andy's features ever so slightly - the realization dawning upon him just how selfish he's being, how much he really cares about Woody, and how much he just doesn't want to let him go.
But the music, oh god the music. The softhearted, one-tone background music shifts slightly, and yet utterly BRUTALLY changes the tone of the entire moment. I pretty much lost it at that point.
It's really at that point that made me realize how far Pixar has evolved in its animation. 10 years ago, that level of facial detail would be pretty much impossible to achieve in their movies. Yet with Wall-E and now Toy Story 3, they've shown to me they can contend with even the best of films in their silent moments.