In particular, two miracles brought Yisro to convert. Rashi (18:1) writes that Yisro heard about kriyas Yam Suf and the war against Amalek, which inspired him to come. Several explanations are given for how milchemes Amalek was the catalyst for Yisro to join the Jewish nation. The holy sefarim explain that after kriyas Yam Suf, Yisro recognized Hashem, but thought he could serve Hashem from the distance in Midyan. He was convinced that the inspiration and awareness of Hashem that he attained would last forever. But soon after the wonders of kriyas Yam Suf, Amalek came and attacked the Jewish nation. When this happened, Yisro realized that seeing miracles isn't sufficient to remain firm in one's belief. One can quickly forget, just as the Amaleikim forgot. He realized that if he wanted to live with emunah in Hashem, he needed to join the Jewish nation. He needed to live among people who believed in Hashem, as each person strengthens his fellow man in their belief. Otherwise, it is so easy to forget.
Reb Shlomo Brevda zt'l once slipped on a stairway in Yerushalayim and miraculously managed to grab onto the railing, which saved him. Some days afterward, Reb Shlomo told the Chazon Ish zt'l that he thought that he would never be the same after his miracle, but he has returned to his old self. The Chazon Ish replied that people tend to forget.
The Or HaChaim (Devarim 8:18) writes, "The yetzer hara's main ploy is to cause people to forget, which destroys them. This can be the translation of the next pasuk, והיה שכוח אם, 'If you will forget that Hashem does kindness to you, אלקיך 'ה תשכח, you will forget Hashem...'"
Reb Shmuel Tchuliner zt'l (a student of Reb Moshe of Kobrin zt'l) once experienced a certain chesed from Hashem and didn't stop talking about it. His family asked him why he kept on repeating the same story. Reb Shmuel replied, "The Or HaChaim says 'The yetzer hara's first ploy is to cause a person to forget Hashem's kindness,' therefore I'm repeating it many times so I won't forget."
That the Amaleikim tried to harm the Yidden so soon after the miracles of kriyas Yam Suf taught Yisro that he too can forget, so he came and joined the Jewish nation, a nation that remembers.
Unity as Strength
In general, it is important to be among other Yidden because unity is our strength and ammunition against the yetzer hara. When the Jewish nation came to Har Sinai, they encamped in unity at the foot of Har Sinai, as it states (19:2) ההר נגד ישראל שם ויחן, "Yisrael encamped there opposite the mountain." Rashi teaches that ויחן is in singular tense to indicate that the nation was united.
They encamped in unity before the mountain. Reb Akiva Eigar zt'l brings the Gemara (Succah 52.), which calls the yetzer hara הר, mountain. So, ההר נגד ישראל שם ויחן, means that when the Jewish nation is united, ההר נגד, that is their strength against the yetzer hara.
Yisro recognized this and joined the Jewish nation. That was his hope to remain strong with emunah in Hashem and overcome the yetzer hara.
