The romantic love that is often evident between spouses is not something that is absolutely a necesarry part of marriage - surely a beautiful thing, and a helpful aid, but not necessary, especially since it is an earthly thing. The love that really matters is the love of the spouse (in the spiritual sense) for the love of God. Marriage is for the procreation of children and the attainment of heaven, all the others frills (personal happiness in marriage "twu wuv" isn't required. My point is that I don't think that is a good comparison. And the notion that we keep the comomandments as a matter of love is protestant - they believe that one shows that they are saved by keeping the commandments, b/c they are so grateful for what Our Lord did for them.
I'm not really understanding where you get your definitions - i don't think of loving God as being the same as freely choosing to obey Him. I think when people speak of the difference between the old law and the new as love they mean that it was necesarry for the old law to be eye for an eye and presenting a stern God, who loves but is nevertheless very exacting. This was necessary b/c there was no Redeemer, the only path to heaven was to do exactly what God said without any defiance. There was no confession, no sacramentals, there were none of the helpful aids of the Church. So Our Merciful Lord, founds the Church with all it does to help us attain heaven - the Redemption is a merciful act: For God so loved the world... and the new law is a merciful law, not rigorous and exacting like the old one.