The Baal Shem Tov says that one of the reasons why people's prayers are not accepted is that they only pray for sorrow to be removed from them, and they do not contemplate that the one who brings sorrow upon them is their loving Father, and He does this only to awaken them to call to Him:
"I heard a parable from the Baal Shem Tov of blessed memory: When suffering comes to a person, let him think of this parable:
A father who has a small son changes his clothes in order to frighten his son and instill fear in him. When the son knows and understands that it is his father, and calls to him 'my father,' He becomes filled with mercy for him, removes his strange clothes, shows himself before his son, and removes the sorrow. And so it is with a person - when some sorrow comes to him, GD forbid, if he understands in his mind that there is nothing besides Him, blessed be His name, and He is frightening him with this sorrow to see if he will have the knowledge to call to GD, or will be immersed in the sorrow. But when he does not have this knowledge, let him know that everything is from Him, blessed be His name, and he seeks medical treatment and physical actions to save from this sorrow, then GD forbid that he remains in sorrow."(בעש"ט עה"ת עמוד התפילה ממ"ח קמחת בשם אורח לחיים מספר טל החיים)
Rebbe Elazar once told the Koznitzer Maggid that his father, Rebbe Elimelech, once told him to be scrupulously careful in melaveh malka
"I told my father that it was hard for me to eat on Motza'ei Shabbos. However, he promised me that whatever I would wish to achieve, I would succeed in doing if I made sure to eat melave malkah.
Mipeninei Noam Elimelch
