If you do not obey the Lord your God

Deuteronomy 28:15-26

However, if you do not obey the Lord your God and do not carefully follow all his commands and decrees I am giving you today, all these curses will come on you and overtake you:

 You will be cursed when you come in and cursed when you go out.

The Lord will send on you curses, confusion and rebuke in everything you put your hand to, until you are destroyed and come to sudden ruin because of the evil you have done in forsaking him. The Lord will plague you with diseases until he has destroyed you from the land you are entering to possess. The Lord will strike you with wasting disease, with fever and inflammation, with scorching heat and drought, with blight and mildew, which will plague you until you perish.

The Lord will cause you to be defeated before your enemies. 

The remainder of the chapter spells out the consequences of disobedience. Have the same rhythmic pattern in Hebrew as 3-6. The curses are opposites of the blessings: disease, famine, defeat, subservience, and ultimately exile, loss of homeland, and all the joys of life.

Why?

The curses are opposites of the blessing, so the people of Israel need to choose which one they like. It is a warning and reminder. The consequence of their behavior is to obey the command of the Lord or disobey it.

How?

The Lord needs us to obey and honor his name from our heart, it is not obeying in the mouth.