It will never last. The Doctor knows this, the TARDIS knows this but the companion is still in awe. They always are, the TARDIS knows. It takes them time to realize that the man or woman they are falling in love with will never stop for them. Even the ones more special to the Doctor are never around for long. River thinks the Doctor will never love her, the TARDIS lets her drive her because they do. The Doctor thinks they will love her forever, the TARDIS knows they will but eventually that fierce romantic love will fade into something else and when they meet again it will not be how either expects.