What is the first part of this verse saying about divorce?

As with Malachi 2:15, it must be said that the Hebrew in the first part of verse 16 is difficult to translate with any certainty. Literally, the Hebrew contains two verbs: the finite verb he hates and the infinitive to divorce. There is no expressed subject. The fact that these two verbs are followed by the clause, says the Lord, the God of Israel incites many to make the Lord the subject of the verb he hates. The resulting translation is, The Lord God of Israel says that he hates divorce. If this is the correct translation, then the Lord is stating his general attitude towards divorce. This does not mean, however, that all divorce is wrong; even God himself divorced Israel (Jeremiah 3:8). Nevertheless, even though divorce is necessary at times to deal with an unfaithful spouse, God takes no pleasure in it. He loathes it. It is contrary to his original design at creation (Matthew 19:4–6).

However, the subject of the verb he hates may remain unspecified, with the resulting translation whoever hates or the man who hates. If this is the correct translation, then the text expresses not the Lord's attitude toward divorce but the attitude of a man toward his wife, which leads him to divorce his wife. The translation in this case would be, If a man divorces [his wife] because he hates [her]… If this is the correct translation, then only divorce on the grounds of hatred is being addressed in our text.

If the original word in verse 16 means hate, why does the ESV translate it as does not love? In the English language, the word hate expresses a radical aversion toward someone or something. But the word for hate in Hebrew does not necessarily convey such a strong aversion. In Hebrew, the word hate can mean love less. For example, in Genesis 29:31 we are told that Leah was hated by Jacob. But it's clear from the historical context that Jacob did not loathe Leah, or that he had a strong aversion toward her. As the preceding verse indicates (Genesis 29:30) Jacob loved Leah less than Rachel. So to hate can mean to love less. In this verse, the translators of the ESV likely felt that the English word hate conveys too strong an emotion. It is not likely that the Jewish men who divorced their wives actually and utterly abhorred or detested their wives. It is more likely that they simply failed to love their wives as they ought, or that they loved the pagan women more than their own wives.