In those two pairs of phrases, 'can't' and 'don't' do indeed have the same meaning. 'Money doesn't buy happiness' = this never happens 'Money can't buy happiness' = it is impossible for this to happen So, in effect, both sentences amount to the same thing. Likewise, 'I can't understand' and 'I don't understand' : if it's not possible for you to understand something, then you don't understand it. However, there are, of course, many cases where 'can't' and 'don't' describe different situations. For example, there's a difference between 'I can't drive' and 'I don't drive', or 'He can't help me' and 'He doesn't help me'.

Money doesn't buy you happiness = it is not the characteristic or quality of money to provide you with happiness. Money can't buy you happiness = money may be able to do many things, but one of the things it is incapable of doing is providing happiness. Think about this: Money can't buy you happiness, but neither can poverty.

"Money can't buy you happiness" is a saying, or 'stock phrase' - it's used as a cliche when talking about people who are so obsessed by money that they forget about their family and friends. The idea is, it doesn't matter how much money you have, you can't use it to buy the love and companionship that comes from spending time with you friends and family.