Entreat me not to leave thee.  Why was Ruth so determined to stay with her mother-in-law?

*catching you so you can't leave*

Avarice.  She had long been plotting to glean.  That is why she killed her husband and brother in-law.  With the return to Judah her plans had achieved their aim, and she began gleaning obsessively.

Steel v Houghton (1788) put an end to this pernicious practice in England and Wales,  where it was held that granting a right to glean would "raise the insolence of the poor"

She loved Naomi and her God with all her heart. 

She loved her very much

she loved her and wanted to make sure she was ok

Ruth 1:16 
And Ruth said, Intreat me not to leave thee... (for) thy God (shall be) my God:

probably because she had no where else to go

When a woman married in those days they married the husband's family as well.  Besides, Ruth didn't see any future in returning to her birth family.

It had more to do with God than it did with Naomi, although Naomi was very important to Ruth. 
 
Naomi was basically the only family Ruth had left AND Ruth became committed to Yahweh when she married.

maybe she just wanted to stay with her