One of the Rishonim, Rav Dovid ben Rav Levi of Narvona [lived in Provence in southeast France in the 13th century] explained why we are so happy in his ספר המכתם.
The Rambam writes in hilchos purim in explaining why מתנות לאביונים is so important: שאין שם שמחה גדולה ומפוארה אלא לשמח לב אביונים ויתומים
אלמנות וגרים שהמשמח לב אומללים האלו דומה לשכינה שנאמר להחיות רוח שפלים ולהחיות לב נדכאים
There is no greater or more glorious simcha than gladdening the hearts of the poor, orphans, widows and converts. For one who gladdens these unfortunates is LIKE THE SHECHINA as it says about [Hashem] "He infuses spirit in the low and gives life to the downtrodden."
The greatest happiness is being like Hashem and the way to do that is to raise the spirits of those who are down. Says the ספר המכתם - don't wait until Purim comes to have the simcha of being like Hashem. Start already on Rosh Chodesh!
Pilei Pla'os - Mamesh sounds like a chasidishe vort. But the chasidim didn't come around until hundreds of years after his passing.
A rishon telling us how to be happy - help make others happy. That's the מרבים בשמחה of Adar.
It's בדוק ומנוסה - time tested. It works.
Get ready tyere precious Yidden!:-)
