המאבד נפש אחת מישראל, מעלה עליו הכתוב כאלו אבד עולם מלא. וכל המקים נפש אחת מישראל, מעלה עליו
Anyone who destroys a life is considered by Scripture to have destroyed an entire world; any any who saves a life is as if he saved an entire world.
צָרִיךְ כָּל אָדָם שֶׁיִּרְאֶה עַצְמוֹ כָּל הַשָּׁנָה כֻּלָּהּ כְּאִלּוּ חֶצְיוֹ זַכַּאי וְחֶצְיוֹ חַיָּב. וְכֵן כָּל הָעוֹלָם חֶצְיוֹ זַכַּאי וְחֶצְיוֹ חַיָּב. חָטָא חֵטְא אֶחָד הֲרֵי הִכְרִיעַ אֶת עַצְמוֹ וְאֶת כָּל הָעוֹלָם כֻּלּוֹ לְכַף חוֹבָה וְגָרַם לוֹ הַשְׁחָתָה. עָשָׂה מִצְוָה אַחַת הֲרֵי הִכְרִיעַ אֶת עַצְמוֹ וְאֶת כָּל הָעוֹלָם כֻּלּוֹ לְכַף זְכוּת וְגָרַם לוֹ וְלָהֶם תְּשׁוּעָה וְהַצָּלָה
Therefore a person needs to see himself all year long as if he is half innocent and half guilty, and also [see] the whole world - half innocent and half guilty. If he sins one sin - he has tilted herself and the whole world to the side of guilt and caused its destruction. If he does one mitzvah - he has tilted herself and the whole world to the side of innocence and caused redemption and rescue.
Ramban on Devarim 30:11
God wants Israel to remember that even in the very depths of exile, teshuvah is not too hard, not far off from you, but is rather 'very close to you,' such that you can do it at any time and in any place.
Simcha Bunim of Peshischa
Everyone must have two pockets, with a note in each pocket, so that he or she can reach into the one or the other, depending on the need. When feeling lowly and depressed, discouraged or disconsolate, one should reach into the right pocket, and, there, find the words: "For my sake was the world created." But when feeling high and mighty one should reach into the left pocket, and find the words: "I am but dust and ashes.