Why do boys giggle?

Silly adolescent reaction in both boys and girls. Very often they themselves don’t quite know what is funny!

Are you a boy or a girl?
That might make all the difference. haha

The boys themselves may have been embarrassed at the sight of you without a shirt. In the nature of boys and men everywhere, they were probably also showing off for one another, too, by making comments about you to each other. Individually, this seems to be less common, but among boys adolescent older males everywhere in groups we can be tough to put up with. I don’t see how any woman does, to be honest.

Because both boys and girls can be very insensitive. From several of your other questions, (Oh I see @gailcalled just listed them) you’ve mentioned that you have a rather small build for your age, and that you suffer from chest acne.

Unfortunately a lot of teenagers, male and female, don’t have a lot of empathy for other people and they don’t understand how painful it can be for someone who doesn’t fit in, or look like, the rest of the crowd.

Do you mind if I ask your age?

Two questions about being laughed at with your shirt off? Well, put your damn shirt on. I don’t walk around topless and I don’t have this problem.
Even Justin Beiber gets made fun of for always going shirtless. It’s kind of stupid to run around like that. There is a time and a place.

Because being shirtless can be read as sexual, and sexuality makes young boys uncomfortable, and when people are uncomfortable, they giggle.

Do what you want in regards to whether or not you wear a shirt. It’s unfair that men should be allowed to not wear shirts and women are told they have to.