The belief of individualism is having the power to make your own decisions and not feel guilty about making that decision.I believe society and its institutions, particularly organized religion and political parties, corrupt the purity of the individual. Other people should believe in individualism because it gives you the freedoms you deserve. It also takes away from the power that the government could possibly have on you, if everybody bought into the system of individualism then the government couldn't argue with it. It gives your children the space to think creatively and grow up how they want.
The transcendentalists didn't always agree on their interpretation of idealism but the main part of it is to create a utopia. I believe this can be created if people lived correctly. If everyone lives correctly it would be much easier to make the society of the utopia. Other people should believe this because when we all work together for a cause it is way easier to obtain that goal or expectation you were looking for. Who doesn't want a perfect society? Oh yeah nobody because it would be the best to live in. But it will also be very difficult to achieve if everybody doesn't try their hardest.