Why a believing Muslim must always be watchful of own deeds in this world?
It is narrated from the first Shia Imam, Ali ibn Abitaleb (AS) who said: Allah (SWT) has hidden four issues in another four:
God has hidden Divine Satisfaction in all worshipping deeds. Therefore, do not count as small any worshipping act and good deed, maybe God’s Satisfaction is in that act/deed and you are unaware of it.
God has hidden Divine Anger in all the sins. Therefore, do not count as small any sin, maybe God’s Anger is in that small sin and you are unaware of it.
God has hidden Divine Acceptance in all Duas. Therefore, do not count as small any Dua, maybe God’s acceptance is assigned in that Dua and you are unaware of it.
Allah (SWT) has hidden God’s Friends among other people. Therefore, do not belittle any servant of God, maybe that person is a God’s Friend and you are unaware of it [1].
[1] Khisal, Sheikh Sadooq (RA), Vol. 1, Page 198.