You are on opening lead, but step into partner's shoes at trick 2.
The opening lead was a small heart around to declarer's ten.
Declarer plays a spade to the king. Do you duck or take it?
If you duck, declarer now plays a diamond to the ten, which wins, when you duck. And you can't beat 3N anymore.
If the opening lead hadn't been a heart, you would have the luxury of smooth ducking your spade ace.
Plus... it's free!