Rabbi Herschel Schachter
Rambam, in Sefer Hamitzvos, (3rd mitzvas aseh) quotes the Talmudic comment (Yoma 86a) on the pasuk, "Veahavta es Hashem elokecha" (Devarim 6:5), that the way to demonstrate our love for Hashem is by inspiring other people to love Him as well, just as one who loves another person will praise him in public and try to motivate others to like him as well. The mitzvah of ahavas Hashem belongs to both the list of chovos halevavos and to the category of chovos haevarim.
In the beginning of Parshas Lech Lecha we read about the "nefesh asher asu becharan," the many individuals whom Avrohom and Sara had brought to believe in Monotheism.
Rambam (first perek in Hilchos Avodah Zarah) quotes a medrash that states that Avraham had tens of thousands of followers. He was very outgoing, and the prophet Michah (7:20) singles him out for his middas hachessed, "Titen emes leyaakov...". By showering much love upon others he affected people so that they came to love G-d as well.
Nevertheless, after the Torah relates the story of the akeidah, G-d's comment to Avrohom is mentioned, "Now I know that you are a God fearing individual." If Avrohom would have served G-d through love alone, he would not have been able to withstand the tremendous emotional strain of the akeidah. It was only because his love of G-d was coupled with a fear of Him that he succeeded in fulfilling this divine command (the Zohar develops this theme and uses the expression of "blending fire with water" fire is a reference to the fear of G-d, while water is an allusion to the love of god).
These two emotions are not mutually exclusive. There is no contradiction between them. Generally speaking, fear is a very unhealthy feeling. But fear of G-d is not unhealthy. The pasuk in Mishlei (10:27) tells us that, "Yirat Hashem tosif yamim ushnot reshaim taktzirena," that fear of G-d will add years to our lives. We are all obligated to both of these biblical mitzvos, both to love G-d and to fear Him at the SAME TIME. The extent that one emphasizes one emotion over the other is a matter of one's personality. Each individual must figure out his own balance.
Yitzchak Avinu was known for his fear of G-d. (See Bereishis 31:53, where Yaakov refers to G-d as "pachad Yitzchak", "the One whom my father feared.") But again, this does not mean that Yitzchak DID NOT POSSESS love of G-d AS WELL.
The opening pasuk in Parshas Vayeshev states that Yaakov lived in Eretz Canaan, the land of megurei aviv, where his father (Yitzchak) had lived. The rabbis of the medrash add another level of interpretation to that phrase. They understood the expression, "megurei" to be rooted in the word, "gerus" - conversion. Just as Avraham engaged in mass proselytizing, so too did Yitzchak after him. True, it was not to the same extent as Avraham, as Yitzchak did not seem to have tens of thousands of followers. Though Yitzchak's dominant emotion was fear, he still possessed a blend with love of G-d in that ha also was involved in proselytizing, as is required of all Jews. Without possessing both of these emotions, we would not be able to fulfill all of the mitzvos.
In his collection of teshuvos entitled, "Meshiv Davar", the Netziv has an essay on the topic of right-wing, left-wing, and centrist Judaism. He explains that whoever does not keep all of the mitzvos is not acting in accordance with the teachings of Judaism. The three groups of Jews - acting as Jews- are divided in accordance with how they strike a balance between fear and love of G-d. Some follow Avraham, and place the emphasis on chessed and ahavas Hashem. Others follow Yitzchak, placing their emphasis on midas hayirah. The centrists are those who attempt to maintain more of a balance between the two emotions without emphasizing one or the other.