Rabbi Abraham Joshua Heschel
As civilization advances, the sense of wonder almost necessarily declines. Such decline is an alarming symptom of our state of mind. Mankind will not perish for want of information; but only for want of appreciation. The beginning of our happiness lies in the understanding that life without wonder is not worth living. What we lack is not a will to believe but a will to wonder.
מצות השמחה בחלקו שנצטוינו עליה במאמר ושמחת בכל הטוב אשר נתן לך ה' אלהיך
There is a specific commandment that obligates us to rejoice with what we have, and that is the verse that states, "And you shall rejoice with all of the good that God has given you" (Devarim 26:11).
על ידי שאדם שמח, על ידי זה הוא יכול להחיות אדם אחר שהוא דבר גדול מאד לשמח לבב בני ישראל, כי רוב בני אדם מלאים יסורים ודאגות וצרות...וכשבא אדם עם פנים שוחקות יכול להחיות אותם ממש והוא דבר גדול מאד.
Likutei Eitzon (Rebbe Nachman), Simchah, 38
When a person is happy he can bring life to other people. Most people are full of worries and troubles...and when someone comes along with a beaming countenance he can literally revive him, and this is a very great thing.
אֵיזֶהוּ עָשִׁיר, הַשָּׂמֵחַ בְּחֶלְקוֹ, שֶׁנֶּאֱמַר (תהלים קכח) יְגִיעַ כַּפֶּיךָ כִּי תֹאכֵל אַשְׁרֶיךָ וְטוֹב לָךְ. אַשְׁרֶיךָ, בָּעוֹלָם הַזֶּה. וְטוֹב לָךְ, לָעוֹלָם הַבָּא.
Who is the rich one? He who is happy with his lot, as it says, "When you eat [from] the work of your hands, you will be happy, and it will be well with you" (Psalms 128:2). "You will be happy" in this world, and "it will be well with you" in the world to come.
לא ישמח אדם בין הבוכים ולא יבכה בין השמחים.
One who is happy should not show it amongst those who are crying, and one should not cry amongst those who are happy.
Soren Kierkegaard, Journals and Papers, 2179
It takes moral courage to grieve, it takes religious courage to rejoice.