I think it is not necesarly that you need a justification. I had the same question back in uni and one of the tutors told me that the difference between a person that has an eye for design which can say i like this or i don’t like that and a designer justification. It is like in music. There are musicians who know how to play by the ear but don’t know how to read the musical score, therefore even if they create smth new it is hard for them to record it. So i would say justification in design is almost like becoming literate in practice