And protons indeed decay because they are composite. It is the interactions and transformations of these quarks which permits the proton to occasionally decay. So let us talk about quarks. The heaviest quarks are the bottom quark with more than 5 times the proton's mass and the as yet unobserved top quark. As I write this I have just heard a rumor that physicists at CERN laboratory in Switzerland have found the top quark and determined its mass to be about 34 times greater than the mass of the proton.
Hadrons are particles which respond to the strong force.
The quark model brought order to this area by demonstrating that all of the many hadron particles were made of two or three quarks. No fractionally charged particles are allowed. Single quarks cannot be found in isolation. The strong force that holds quarks together is very strong indeed. It is so strong that if you try to pull a quark out of a proton, you have to pull very hard, supplying in the process a large amount of energy.