Meaning of "See You When You're 40" by Dido

The song describes the journey of realizing that someone who seemed complicated and mysterious is really just full of empty words and lacking true depth. The protagonist recognizes the warning signs of someone who is not genuine or mature enough to have a meaningful connection. The decision to leave before wasting more time with this person is based on the realization that they will never change and are not worth investing in. The lyrics also touch on the idea of not wanting to be associated with someone who thrives on misery and sadness, choosing to walk away and move on from that toxic relationship.