Why do I do the things that I don't Desire to do?

"For I do not understand my own actions. For I do not do what I want, but I do the very thing I hate." - Romans 7:15

Why do people have to fight obesity? Why do people have to fight laziness?

Because their desires and their actions conflict. 

They desire to be thin and they desire to be disciplined, but they aren't, and often, they aren't successfully pursuing these goals.

What could an explanation be for this phenomenon?

It could be that it is naive generosity to assume that an obese person actually wants to be thin. Perhaps their greater desire is for the pleasure food brings, not so much for the health or wellbeing that thinness brings.  Though I think that anyone in this position, if asked, would strongly agree that they desire to be thin, but they seem to lack the capacity to make it happen.

Here it is critical to distinguish between our cognitive desires (that which we desire cognitively) and our sensational desires (that which we desire because it brings pleasure to our senses). The desires that are often at conflict is between that which we are sensationally addicted to and that which we cognitively desire. 

For example, I sensationally desire food because it feels good, but I desire to exercise because I am cognitively aware that it will make me feel even better than the food in the end. 

When God created us, he must have created these two facets of human desire (Cognitive and Sensational). If we look at Genesis 3, we find the first struggle between these two facets of desire:

Now the serpent was more crafty than any other beast of the field that the Lord God had made. He said to the woman, “Did God actually say, ‘You shall not eat of any tree in the garden’?” And the woman said to the serpent, “We may eat of the fruit of the trees in the garden, but God said, ‘You shall not eat of the fruit of the tree that is in the midst of the garden, neither shall you touch it, lest you die.’” But the serpent said to the woman, “You will not surely die. For God knows that when you eat of it your eyes will be opened, and you will be like God, knowing good and evil.” So when the woman saw that the tree was good for food, and that it was a delight to the eyes, and that the tree was to be desired to make one wise, she took of its fruit and ate, and she also gave some to her husband who was with her, and he ate. 

Genesis 3:1-6

The text above says that when Eve "saw that the tree was good for food, and that it was a delight to the eyes." Right here, we have sensational desire. It continues and says, "the woman saw...the tree was to be desired to make one wise." Right here, we seem to have cognitive desire. 

Now, in a perfect world, these facets of desire would necessarily be perfectly aligned with each-other -- there would be no struggle between what we cognitively desire and what we sensationally desire. In other words, the sensational desire to be in God's presence would overwhelm all other desires and we would cognitively be aware that this is the healthiest option for our lives as well. 

Adam and even failed to keep both of these desires for God, but Jesus does not. Jesus is tempted by the devil in the wilderness, after forty days of not eating. The devil says, 

“If you are the Son of God, command this stone to become bread.” And Jesus answered him, “It is written, ‘Man shall not live by bread alone.’”  - Luke 4:3-4

Jesus essentially says, "yes, I desire bread, but there is more to life than bread." 

Here's what's fascinating, I would argue that Jesus' sensational desire was for bread, yes, but his greater sensational desire was for the presence of his heavenly Father. 

So the question isn't: is your only sensational desire for God?

No.

The question is: does your sensational desire for God overwhelm all other sensational desires?

Do we enjoy the presence of God above everything else?

Many people will say that they cognitively desire the presence of God -- that is, they believe that a relationship with God is healthy for them. But only those who really love God will desire God, not for what they can get from the relationship, but for what the relationship creates:

The constant presence of God in your life. 

So...

Does God satisfy you more than anything else?