based on Siach Shaul, p. 65-66
Chazal tell us that Eliezer, the servant of Avraham was a respected Torah scholar within the circle of Avraham’s disciples (Yoma 28b). The Torah (Bereisheet 24:2) calls him "the venerable one of his home, who ruled over all that was his," which the gemara says refers to all of Avraham’s Torah. Chazal also say that the name Damesek Eliezer (ibid. 15:2) hints that he was doleh u’mashkeh (drew and gave to drink) his master’s Torah to others. The midrash (Bereisheet Rabba 59:8) says that his mastery extended to his control over his evil inclination. Indeed, it is no surprise that the Rabbis said in the context of this great man: "The speech of the servants of the forefathers’ house is nicer than the teachings of the children" (ibid. 60:8).
Yet, we find a starkly different teaching about Eliezer in regard to the pasuk, "In the hands of C’na’an are scales of trickery, to deny his dear friend" (Hoshea 12:8). The midrash (Bereisheet Rabba 59:9) says that Eliezer was "weighing" his daughter as being fitting for Yitzchak. Avraham told him: "You are cursed (as you come from Cham), and my son is blessed, and it is not fitting for a cursed person to cling to a blessed one." The question needs to be asked: after seeing all the wonderful things said about Eliezer, how could he be considered cursed, and why was his daughter not fit to marry the blessed Yitzchak?
If we look at the difference in behavior of master and servant during this episode, we can find a hint of the difference in their character and thinking. Eliezer asked Avraham: "Maybe she [the appointed girl] will not want to follow me" (Bereisheet 24:5). In contrast, Avraham said that Hashem "will send His angel before you" (ibid. 7). Avraham had no doubts; he had a clear path and firm belief. He had no questions and needed no answers. He knew that the local population of girls did not include an appropriate wife and that the search had to be taken up at a distance. He knew the correct person was there, and so he was ready to wait for the sign of who she was.
Eliezer was different. While he was dedicated to his master, he was uncertain and felt a need to consider other options. The plan had to be "realistic" and calculated. At first glance, Eliezer seems correct. We are not usually supposed to rely on miracles. However, in practice, the entire process of finding a mate for Yitzchak worked miraculously, and it became clear that a "miraculous reality" is a totally different one. Avraham and his confidence in Hashem was correct; Eliezer’s suspicion was not.
In order to put the differences of approaches in perspective, the Torah hints with unusual spelling and Chazal (Yalkut Shimoni, Heshea 12) deciphered as follows: "To me - if the woman will not follow me." Whereas Avraham was able to elevate himself over his own personal calculations, Eliezer had a mixture of considerations. He indeed was an expert student and exponent of his master’s teachings, but he still allowed his own personal agendas to creep in. From there stemmed his doubt and his shortcoming.