A smile because the nights are short!
 And every morning brings such pleasure
Of sweet love-making, harmless sport:
 Love that makes and finds its treasure;
The opening is abrupt and spare and fast-paced. The sentence has no verb — the speaker seems not to have the time — and the exclamation mark emphasises her happiness. The nights are short because the speaker is loved and can smile. 
 Note that the first line of the second stanza echoes and inverts this.
A Smile And A Sigh