Why Fast?

The first situation that pops up in my mind is from the gospel when the disciples were empowered by Jesus to cast demons but they failed once and they came to Jesus to ask why they were not able to cast that particular demon. Jesus answered that those kinds of demons could be cast out only by fasting and praying.

It is interesting to see that when Jesus spoke about fasting, He said, "when you fast" (Matthew 6:16), referring to it as to an already assumed thing, not as if fasting were an "if". Our Lord assumes that Christians will fast. It is obvious from the New Testament that Jesus disciples along with other Christians did fast.
"But in all things approving ourselves as the ministers of God, in much patience, in afflictions… in fastings" 2 Corinthians 6:4-5

So what does fasting represent after all?

In Joel 2:12 it is a clear term for dedication: "Yet even now," says the Lord, "return to me with all your heart, with fasting. . ." ().God wanted them to do something sincerely, to really mean it.

"When you seek me with all your heart, I will be found by you" (Jeremiah 29:13, 14).Again, here we see that the One who created us can indeed see when we mean "business" and when we fake it. He wants us to really be serious 100% not 99%.

Therefore, fasting is an expression of showing that you truly want to see what God has to say about a specific situation.

When someone is willing to show that he/she means business, that person doesn’t find it difficult to set aside all other legitimate appetites of the body and focus on communicating with God. This demonstrates that such a person seeks God with all his/her heart.

Why fast?

To express the fact that we are seeking God "with all our heart." The focus is set on a more important thing then feeding ourselves physically. It is a physical expression of saying :"All I care about is (name your cause) and I am determined to focus on talking to God about it without being interrupted by food or things of this life".

In addition, when you put God’s priorities first, that shows humility. Fasting also means "to humble oneself before the Lord" (Psalm 35:13).

We must keep in mind that we do have a loving God that wants (even more then we do) to give us the best things for our lives. Yet, from Moses up to nowadays fasting and prayer are emphasized in the Bible and are regarded as the usual and normal practice in the Christian life.

Can we manipulate God by fasting?

If you think that by not eating -a day or two or as long as you wish- you can twist God’s arm determining Him to do something the way you think it should be done, you probably don’t realize that manipulating God doesn’t work and it is not possible.

No one can press God! A group of people in the book of Acts tried to get God on their side by manipulative fasting: "In the morning some of the Jews made a plan to kill Paul, and they took an oath not to eat or drink anything until they had killed him. They went to the leading priests and the older Jewish leaders and said, 'We have taken an oath not to eat or drink until we have killed Paul'" (Acts 23:12,14). However, God did not listened to their prayer and their plan did not succeed.

Using fasting to manipulate God was attempted in Jeremiah's time as well. God said, "Although they fast, I will not listen to their cry; though they offer burn offerings and grain offering, I will not accept them. I will destroy them with the sword, famine, and plague" (Jeremiah 14:12).

Manipulating God by fasting doesn’t work! We don’t fast so we could make God do things our way, but on the contrary, to ask God to open our eyes to do the things His way. He has a better view you know, He sees things from an eternal perspective and He knows all the consequences of the choices we make. That’s why, it is smart to ask Him about the best choice. Fasting is not a hunger strike to force God work our way!

Also fasting is not another "weight loss diet" or a body discipline. Fasting core and focus is the soul, although body is involved.
Isaiah 58: 3-7 says:

'Why have we fasted,' they say, 'and you have not seen it? Why have we humbled ourselves, and you have not noticed’

"Yet on the day of your fasting, you do as you please and exploit all your workers. Your fasting ends in quarreling and strife, and in striking each other with wicked fists. You cannot fast as you do today and expect your voice to be heard on high.

"Is not this the kind of fasting I have chosen: to loose the chains of injustice and untie the cords of the yoke, to set the oppressed free and break every yoke?

Is it not to share your food with the hungry and to provide the poor wanderer with shelter- when you see the naked, to clothe him, and not to turn away from your own flesh and blood?

God sees all motivations of our hearts, may we have His love as motivation and then, our fasting will be heard on high!