The mostimportant thing a Jew needs is to give praise and to thank Hashem for all the kindnesses that He does with us and everything that He does is only to do a favor for us, even if we do not understand, because everything is with an exact calculation. I thought that this is pshat in Posuk )יוֹשֵׁב בַשָּׁמַיִם יִשְׂחָק (תהלים בד "He who sits in Heaven laughs"—that the Almighty laughs when people wonder why things happen, but in Heaven it is known that this is with an exact plan and for the good. And because the Almighty loves every Jew, we must go in His ways and also love every Jew. And it is written that Rosh Hashana is יְוֹם תְרוּעָה which תְרוּעָה means an expression of friendship, unity ריעות. On the night of Rosh Hashanah, the custom is to go around the shul and wish your fellow congregants "לשנה טובה תכתב ותחתם", "May you be inscribed and sealed for a good year." Let it be known that two angels accompany a Yid and they listen in as he wishes his friends warm hearted with friendship and they see unity among Klal Yisroel, this inspires them to ascend to Heaven and plead for a good and sweet year for Klal Yisroel Therefore, we wish right away on Rosh Hashanah night for everyone Leshana Tova, to show that we love everyone, and when we will have Ahavat Yisrael, the Almighty will also love us and write us down for a good year.
This should prod one to have Ahavas Yisrael and wish all full hearted. By doing so it is a cinch to have a good and prosperous year. We can expand on this insight a step further as David Hamelech states in Tehilim 91:11 י :מַלְאָכָיו יְצַוָּה לֶךְ לִשְׁמָרְךָ בְּכָל דְּרָכֶיךָ For He will command His angels on your behalf to guard you in all your ways. The angels are for our benefit and are also our prosecutors. They also benefit from our good deeds. However on the day of Judgement, these angels that guarded you all along will attest to your behavior. By doing good deeds and helping a fellow Yid and praying for their good welfare, these angels will guard you on this world as well at the time of need at the Heavenly court. (Yehuda Z. Klitnick)