Allah the Almighty says:

﴾ Nay! Verily, man does transgress all bounds (in disbelief and evil deed, etc.)* Because he considers himself self-sufficient.﴿ 

[Al Alaq, 6-7]

Some people are heedless, because they are powerful and rich. Money is affluent in their hands and their orders are listened to, they have their powerful status either in commerce, industry or in the government, and they are served tea or fruit juice every 15 minutes, then they are followed with plate of fruits. They are never hungry or thirsty, and all their physical needs are met with the highest levels of services. 

Such people are heedless of the fact that they are servants of Allah the Almighty and that they need Him in their food and drinks, but the abundance of the latter hinders this fact. Siyaam makes people aware of this fact (being in need of Allah).