Voluntary acts are like presents

In Ibn hajar’s Magnus opum; fathul Bari. In his explanation of the famous hadeeth of wilaya, he رحمة الله عليه mentions something quite profound when elucidating on the reason why voluntary acts are so beloved to Allah.
فقد جرت العادة أن التقرب يكون غالبا بغير ما وجب على المتقرب كالهدية والتحفة بخلاف من يؤدي ما عليه من خراج أو يقضي ما عليه من دين

The trend is that closeness (sought with some one you love) is predominately obtained via voluntary acts, such as giving gifts and presents, which is in contrast to compulsory acts that are expected to be performed, such as tax money and debt. 

End quote

The point being that compulsory acts are expected to be done by everyone but that extra effort is only done by he true lover and Allah سبحانه و تعالى loves to be loved.