Why does Jesus tell the multitudes to count the cost before becoming a disciple?

Because following Jesus (becoming a disciple) requires full commitment. No turning back.

Jesus by asking: “Who of you that wants to build a tower does not first sit down and calculate the expense [or, count the cost], to see if he has enough to complete it?” (Luke 14:28) What cost did Jesus mean?

All Christians encounter trials—some serious. (Psalm 34:19; Matthew 10:36

We must already have figured such challenges into the cost of being a disciple of Christ, knowing that the reward—salvation from sin and death—is worth far more than anything that the present system can offer us. Yes, nothing that God permits—even death—can permanently harm us if we continue to serve him.—2 Corinthians 4:16-18; Philippians 3:8.

When we personally experience Jehovah’s blessing as a result of our faithful course, our faith is strengthened and deepened. In that way, we follow the example of Jesus, his first disciples, and all the men and women of faith who down through the ages correctly ‘counted the cost’ of serving God.

Yes, this is true. Jesus Christ left no doubt about what being his disciple would involve. “A slave is not greater than his master,” he told his followers. “If they have persecuted me, they will persecute you also.” Jesus was hated “without cause.” (John 15:18-20, 25; Psalm 69:4; Luke 23:22) His disciples could expect the same—opposition without justifiable basis. On more than one occasion, he warned them: “You will be objects of hatred.”—Matthew 10:22; 24:9.

Consequently, Jesus urged prospective followers to “count the cost” of discipleship. (Luke 14:28, Revised Standard Version) Why? Not so as to decide whether they should become his followers or not, but so as to be determined to fulfill what is involved. We must be prepared to endure any trials or hardships that come with the privilege. (Luke 14:27) No one forces us to serve God as a follower of Christ. It is a voluntary decision; it is also an informed decision. We know beforehand that in addition to the blessings that we will experience from entering into a dedicated relationship with God, we will be “objects of hatred.” So we are not puzzled when we meet with opposition. We have ‘counted the cost,’ and we are fully prepared to pay it.—1 Peter 4:12-14.

He was telling them that they would have to leave everything else behind in order to follow him - families, jobs, possessions, everything. And the amazing thing is that the apostles did just that.

Mat 6:19 ¶ Lay not up for yourselves treasures upon earth, where moth and rust doth corrupt, and where thieves break through and steal:

Mat 6:20 But lay up for yourselves treasures in heaven, where neither moth nor rust doth corrupt, and where thieves do not break through nor steal:

Mat 6:21 For where your treasure is, there will your heart be also.

because one must also realize that Christ was hated by some and as He said 'know that if they hate you, they also hated me before you" a servant is not greater than his master. so therefore there is the cost of being ridiculed, forsaken by family and friends, etc. and bearing insults and shame all because of the cross and the gospel of salvation.

Jesus said, "...and people will lying say all sorts of things about you..." But the Bible says, in John 17:3 "This means everlasting life taking in knowledge of you, and the only true God, Jehovah."

Jesus gave his life for all of mankind in sacrifice, and many sacrifices might be required of us who become his disciples."

But the rewards are great! This is what is in store for those of mankind who obey Jehovah God--a restored paradise as it was in the beginning:

"In the beginning, God created the heaven and the earth..."

He created a perfect human couple, and Satan deceived them into believing they could be "like" Jehovah God. They sinned and God had warned them that if they did, "...they would surely die..." So all mankind had sin and death passed on to them.

But Jehovah's purpose could not be thwarted--and He purposed " a new heaven and a NEW EARTH in which righteousness is to dwell."

David [whom the Bible refers to as being `a man agreeable to Jehovah's heart'] did not ascend to the heavens." No, but will be resurrected back to a paradise earth.

Scriptures that state a new earth will be created and all God’s followers will live there and  He commanded his disciples to preach his message in all the earth. (Matthew 24:14; 28:19,20.)

God’s For told Kingdom/Government will last forever. 

Psalms 37:10,11,29.

But the meek shall inherit the earth; and shall delight themselves in the abundance of peace.

Isaiah 65:17

for, behold, I create new heavens and a new earth: and the former shall not be remembered, nor come into mind.

Matthew 5:5

Blessed are the meek: for they shall inherit the earth.

Eternal Happiness - In Heaven Or On Earth? 

http://wol.jw.org/en/wol/d/r1/lp-e/20007... 

Taking in Knowledge—Now and Forever :

 http://wol.jw.org/en/wol/d/r1/lp-e/20052... 

You can live forever on earth:

https://www.box.com/s/rfd25irws46ua0y04v...

"Praise Jah, you people!" "God will wipe out every tear from their eyes..."

He wants people to know honestly what they must give. Even their life. He wants to be honest with them. But He says those who lose their life for His sake will find it.

*lost