Jamieson, Fausset and Brown on 1 Samuel 1:11

11. she prayed ... she vowed a vow--Here is a specimen of the intense desire that reigned in the bosoms of the Hebrew women for children. This was the burden of Hannah's prayer; and the strong preference she expressed for a male child originated in her purpose of dedicating him to the tabernacle service. The circumstance of his birth bound him to this; but his residence within the precincts of the sanctuary would have to commence at an earlier age than usual, in consequence of the Nazarite vow.