Or HaChaim on Genesis 29:31/אור החיים על בראשית כ״ט:ל״א

1 וירא ה׳ כי שנואה לאה, G'd saw that Leah was hated, etc. The Torah tells us here that only G'd was aware of the fact that Leah was hated; she herself only felt that she was not beloved as we shall explain by examining her own words presently. Bereshit Rabbah 71,2 sees in the word שנואה merely an allusion that Leah had been slated to become the wife of Esau who was hated (compare Maleachi 1,3 where G'd expressed His hatred for Esau). Accordingly, Leah is here called by the name of the husband originally intended for her. If Leah had not produced children the people would have seen this as proof that she was not meant to be Jacob's wife; this is why G'd opened her womb and granted her children. The Torah goes on to say that Rachel was barren to indicate that it was hoped that by making Leah fruitful Jacob's hatred towards Leah would abate when she could give him children whereas Rachel apparently could not.