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how did 'love' function as a core virtue or concept in 1st century jewish and greco roman thought?

The passage, plainly, with room to sit with it.

Mark 12:30-31

“you shall love the Lord your God with all your heart, and with all your soul, and with all your mind, and with all your strength.’ This is the first commandment. The second is like this, ‘You shall love your neighbor as yourself.’ There is no other commandment greater than these.””

This passage is from the Gospel of Mark, written around 60-75 CE, likely for a Roman audience. Jesus is responding to a question from a scribe about which commandment is the most important, drawing on core teachings from the Jewish Torah (Deuteronomy 6:5 and Leviticus 19:18).

What does 'love' mean in practice when it encompasses both divine devotion and human relationship?

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Read alongside it

Deuteronomy 6:5
The Old Testament command to love God with all one's being, quoted by Jesus.
Leviticus 19:18
The Old Testament command to love one's neighbor as oneself, also quoted by Jesus.
Romans 13:8-10
Paul's teaching that love fulfills the law, echoing Jesus's emphasis.

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