Trunks I'll give you, but the other 2 went through some trauma. Goten having to deal with Chi Chi, and contend with a SSJ in Trunks beating his ass everyday probably had a little something to do with it. Bardock got transported back in time... everything he ever knew, cared for or fought for was just taken from him in an instant. Even if he got back everything would still be destroyed. That's some serious emotional baggage to draw on.
As to why you have to completely go against who you are as a person to reach it, I couldn't say really. Maybe something to with breaking down mental barriers that although cause you to be who you are, only serve to limit the instincts and emotion of the SSJ form. Yet in order to maintain it and not go crazy you have to have enough willpower and knowledge of yourself to strike a balance and direct this massive amount of energy. Like how some saiyans cannot control or remember ape form. That's how I've always viewed it anyway. It's the same reason why Brolly is batshit crazy too, The SSJ rage and passion is all he has ever known. He never had a chance to create an identity for himself that he could stay grounded to while in the form.