Mainly because that is the way it is said. I assume that the reason is that 'good' and 'bad' are short for 'good things' and 'bad things', so you are effectively saying 'the things that are good and the things that are bad...'

Yes,we do. We do need to put "the". What is good and what is bad maybe mentioned before this sentence. For exmaple Waking up early is good for fitness Going to bed late is bad for health. If this is the case,the good = waking up early,the bad = going to bed late We don't say like this. She accepted both waking up early and going to bed late with a calm state of mind. Good and bad are adjectives but in such cases,we put "the" to make them nouns. the + <adjective> = something <adjective> This might indicates the usage of "the" as prefix of adjectives.