At age 75, the Torah records G-d's first interaction with Abraham (formally known as Abram), which was G-d giving Abraham a commandment (Genesis 12:1): לך לך (lech lecha). This commandment was the beginning of Abraham's Ten Trials. Why was it a test for Abraham to "go forth from his native land, from his birthplace, and from his father's house to the land that G-d was to give him?" What made leaving so difficult and arduous?
To answer this question, we should answer another question: Why does the verse say לך לך? It would have been sufficient to say "go" (לך). The second "לך" is seemingly superfluous. Why say לך לך? According to Rashi, G-d said לך לך because that second לך meant "for your benefit." Rashi opined that the benefit was so that Abraham could become rewarded and become a great nation. Not that I disagree with Rashi, but I think there is something more taking place.
The phrase לך לך is used in one other place in the Torah: right before Abraham's Tenth Trial, the one where Abraham is supposed to sacrifice his son (Genesis 22:2). Both passages also have a threefold description that follows לך לך, both passages conclude with similar blessings, and both passages act as brackets for Abraham's narrative, which is to say that there is a connection. Both the first and last of Abraham's trials were arguably his most difficult trials. If we are to take Rashi's interpretation and apply it to both trials, then I can infer that there was to be some sort of benefit from the trial, but what is the benefit?
If the test were truly a benefit, I have to wonder what the test is in the first place. One can argue that leaving one's homeland, community, and family is difficult. If the verse were to only convey that leaving a place to which one is accustomed is strenuous, the verse would have said that Abraham was to leave "from the home of your father, from your birthplace, from your country," not the other way around. The Malbim picked up on the reverse word order, and he concluded that the order is "in reverse" is because the behaviors most heavily ingrained within us take place in the home.
לך לך is not leaving a physical place, but rather one in which you are to "go to yourself," i.e., go to your true self [by serving G-d] (Lubavitcher Rebbe). Going off the Lubavitcher Rebbe's interpretation, לך לך is not a physical departure, but a spiritual one. Since the Torah is oddly silent about Abraham's life prior to this moment, we have to use other texts to fill in the blanks. Midrashic tradition teaches us that Abraham was surrounded by idolatry (Mirdrash Rabbah, Genesis 38:13).
The first step is always the most difficult to take. Even if Abraham intellectually realized that monotheism is correct during his time in Haran, he just couldn't pick up and leave because understanding something from an intellectual standpoint and internalizing it are not the same thing. Introspection and reflection take time. He had to detach himself emotionally from his family and native land first. It takes time to realize what your purpose in life is and who you are meant to be, and that process does not get any easier with age. To leave family, friends, and community, essentially, everything Abraham knew for some unknown land and a newly found religion. He essentially had to become non-conformist. This first step not only takes a good amount of faith in G-d, but it also requires a huge step out of one's comfort zone, which I would argue was Abraham's first test.
As scary as the unknown can be, it is also exhilarating. To paraphrase the Blues Brothers, Abraham was on a mission from G-d. By leaving home, Abraham not only became the patriarch of Judaism, but he established ethical monotheism in this world. Abraham was a man who was so great that he walked in front of G-d. He became Judaism's archetype of חסד (loving-kindness). Abraham realized that change is neither good nor bad, but what you make of it. Upon realizing that, he stepped well outside his comfort zone and took those first steps, and as a result, he became a better human being.