Q: why are the 4th and 5th called perfect but the 2nd separated into major and minor
A: i will explain some fun facts for you, and then you will be able to make a lot of sense out of things: if you invert any interval (swap the high and low note), to get the name of the new interval you subtract the number from 9, and for the quality, major becomes minor, minor becomes major, augmented becomes dininished, diminished becomes augmented, and perfect stays perfect. so for example, C to E is a major 3rd, E to C is a minor 6th. C to G# is an augmented 5th, G# to C is a diminished 4th. C to F is a perfect 4th, F to C is a perfect 5th. C to Gb is a diminished 5th, Gb to C is an augmented 4th. if all goes well, this might raise more questions, but then answer even more