The transciency of relationships
All the characters in WKW films eventually come to accept the fact that all relationships will eventually end
'she will reach her destination' and 'i know i'm getting off soon'
End scene is a good example of this: like the cig smoke 'relationships form like the puff of smoke, but then quickly lose their cohesion and the particles of smoke quickly drift apart.'
Additionally, the shot of the skyscrapers can be interpretated as the transient nature of Hong Kong.