The Ramban in a discussion on the topics of Yiras Hashem and Ahavas Hashem says that Yirah must come first. Only when a person has successfully reached a level of Yiras Hashem can he begin to work on Ahava. It cannot work the other way. One cannot work on Ahava and then work on Yirah.
There is a remez, a clue, to this in the words themselves. The word Yirah is spelled Yud, Resh, Alef, Hey and the word Ahava is spelled Alef, Hey, Bais, Hey. At the end of the word Yirah, you have the Alef and Hey that begins the word Ahava. Whereas, the letters at the end of the word Ahava do not begin the word Yirah. Ahava is a direct continuation of Yirah.
