“Only know you've been high when you're feeling low
Only hate the road when you're missing home”

Passenger – Let Her Go Lyrics on LyricsMode
He implies that you only truly understand something when you actually experiance something 1st hand. He doesent want to think about the things that remind him of what he misses. for example "only hate the road when your missing home" means that he hates thinking of how he would get home simply because by thinking of this makes him miss home more. the thought of these things makes it more painful for him and makes him relize what he took for granted. This is just an annology for us to kinda get a feeling of what its like to take something for granted.