Usmc boot camp?

whenever I talk to a marine about their time in boot camp and what they experienced, they always smile and tell me about how hard it was but how it was worth it as well. But they always hold back, they hold back somethings in their smile. some even tell me, "you'll never know everything until you've lived through it". can someone who has been through boot camp please tell me just what they mean? im not trying to pester, im trying to make a decision about whether or not its a good choice for me and I need all the info I can get for that. Thanks.