We are commanded to love God. Once again, we have to ask the obvious question: how can the Torah tell us how to feel? Let’s answer that by flipping it around.
Do you hate Hitler? If you answered yes, which I’ll assume you did, why do you hate Hitler? You never even met the man! You hate him because you are familiar with his deeds. How can someone not hate a person known to be an evil, genocidal maniac?
It works the other way, too. How can we be commanded to love God? This requires us to familiarize ourselves with Him and His works. If we get to know Him, we won’t be able to help but love Him. This love will then arouse us to enthusiastically inform others all about Him.
The reason for this mitzvah is to move us to better keep the Torah and its laws. A person might do them simply because they were commanded or out of fear of punishment, but these pale before the manner of one who is motivated by a love of God.
This mitzvah applies to both men and women in all times and places. In the Talmud, it is discussed in the tractate of Brachos on pages 54a and 61b. It is codified in the Mishneh Torah in the second chapter of Hilchos Yesodei HaTorah. It is #3 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos and #3 of the 77 positive mitzvos that can be fulfilled today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.