Why did Boaz keep calling Ruth "my daughter"?

Ruth 3:

7 After Boaz had finished eating and drinking and was in good spirits, he went to lie down at the end of the heap of grain. Then Ruth went in secretly, uncovered his feet, and lay down.

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This carried sexual undertones.

8 At midnight, Boaz was startled, turned over, and there lying at his feet was a woman!
9 “Who are you?” he asked.

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Boaz was surprised.

“I am your servant Ruth,” she replied. “Spread the corner of your garment over me, for you are a kinsman-redeemer.”

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More sexual connotation.

10 Then Boaz said, “May the LORD bless you, my daughter. You have shown more kindness now than before, because you have not run after the younger men, whether rich or poor. 11 And now do not be afraid, my daughter. I will do for you whatever you request since all my fellow townspeople know that you are a woman of noble character.

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Strong's Hebrew: 1323. בַּת (bath) — 589 Occurrences

BDB:

1. daughter, female child, born of a woman
   f. used in kindly address, בִּתִּי Ruth 3:10,11 (Boaz to Ruth), compare Psalm 45:11; בְּנוֺתַי in mouth of ׳י Isaiah 43:6 ("" בָּנַי).
2. young women,

The sexual connotation is in some readers' heads but not in Boaz's. Boaz was a man of standing (Ru 2:1) and propriety.

Why did Boaz call Ruth under that circumstance 'my daughter'?

In their culture, it was a term of endearment and closeness. Ruth was looking for help like a helpless daughter. The term reassured her with a sense of promise and protection from a father.

12 Yes, it is true that I am a kinsman-redeemer, but there is a redeemer nearer than I.

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Boaz demonstrated his moral responsibility and honorable intentions toward her.