Rebbe Leibele Eiger zt'l explains that בָא כִּי ֶּׁׁמֶשהַש, the sun setting, was a reminder for Yaakov Avinu that just as the sun sets and then the morning comes, so, too, Yaakov should know that all tzaros will end and good times will come. This was a great chizuk for Yaakov Avinu when he went to Charan, to the house of Lavan the rasha. He was taught that suffering has an end.
We quote from Rebbe Leibele Eiger's holy sefer, Toras Emes: "He knew that all the darkness that he endures in Charan is only for a certain amount of time. The salvation is prepared and ready to come. Only for the time being, the salvation is concealed. It can be compared to when the sun sets. It looks like there isn't a sun in the world anymore, when in truth it just went away for a moment, and it is prepared to shine again when the morning comes. Similarly, Yaakov understood that the darkness is only for a short time, like the sun setting."
Reb Leibele Eiger adds that this is the specialty of tefillas Maariv. It is said when it is dark, but one shouldn't lose hope. The salvation is prepared, and it will come. It will become light again, just as the night turns into the morning.
Chazal (Brachos 4a) say, "Who is a ben Olam HaBa? Someone who connects לתפילה גאולה at Maariv." As we explained, the tefillah of Maariv, at nighttime, hints to the person who feels that the "sun set early" for him, and he is going through difficult times. Nevertheless, he connects לתפילה גאולה. This means he believes that there will be a geulah, redemption. He is a הבא עולם בן. He merits Olam HaBa because he has this emunah.
