How should I make Dua to get closer to Allah and seek what is best for me without being arrogant or demanding? What really does being happy with decree of Allah (swt) mean?
Sincere believers seek from Allah (SWT) to Grant them the best for them according to He Knows, not what they think. Supplications Du'a narrated from the Prophet (SAWA) and Ahlul Bayt (AS) are full of these submissions. We admit that we have no knowledge in front of what Allah Knows about us and our future, that is why, we need to totally submit to His Will and seek from Him to a grant us what He Knows the best for us in this life and hereafter.
Wassalam.