Why BETTER half?

My better half

My husband or my wife.Meaning

Origin

This term wasn't originally restricted to referring to one's spouse as we use it now, but to a dear friend. It was used that way by the Roman poet Horace and later by Statius. The allusion then was to a friend so dear that he/she was more than half of a person's being. That meaning persists, although these days, if the term is used seriously rather than sarcastically, it is generally considered to mean 'the superior half of a married couple'. That is, better in quality rather than in quantity.

Sir Philip Sidney was the first to put into print the use of this phrase to mean spouse, in The Countesse of Pembrokes Arcadia, 1580:

"My deare, my better halfe (sayd hee) I find I must now leaue thee."