Prophesy is our Heavenly Fathers way, through his prophets, of making his mind and will known. He uses prophesy as a witnessing tool to save lost souls. He also gives warnings for people to repent.

Jonah 3:1 And the word of the LORD came unto Jonah the second time, saying,
Jonah 3:2 Arise, go unto Nineveh, that great city, and preach unto it the preaching that I bid thee.
Jonah 3:3 So Jonah arose, and went unto Nineveh, according to the word of the LORD. Now Nineveh was an exceeding great city of three days' journey.
Jonah 3:4 And Jonah began to enter into the city a day's journey, and he cried, and said, Yet forty days, and Nineveh shall be overthrown.
Jonah 3:5 So the people of Nineveh believed God, and proclaimed a fast, and put on sackcloth, from the greatest of them even to the least of them.
Jonah 3:6 For word came unto the king of Nineveh, and he arose from his throne, and he laid his robe from him, and covered him with sackcloth, and sat in ashes.
Jonah 3:7 And he caused it to be proclaimed and published through Nineveh by the decree of the king and his nobles, saying, Let neither man nor beast, herd nor flock, taste any thing: let them not feed, nor drink water:
Jonah 3:8 But let man and beast be covered with sackcloth, and cry mightily unto God: yea, let them turn every one from his evil way, and from the violence that is in their hands.
Jonah 3:9 Who can tell if God will turn and repent, and turn away from his fierce anger, that we perish not?
Jonah 3:10 And God saw their works, that they turned from their evil way; and God repented of the evil, that he had said that he would do unto them; and he did it not.

Beware of false prophets. All that prophesy are not of the Lord.

Jesus himself said;
Mat 7:15 Beware of false prophets, which come to you in sheep's clothing, but inwardly they are ravening wolves.
Mat 7:16 Ye shall know them by their fruits. Do men gather grapes of thorns, or figs of thistles?
Mat 7:17 Even so every good tree bringeth forth good fruit; but a corrupt tree bringeth forth evil fruit.
Mat 7:18 A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit.
Mat 7:19 Every tree that bringeth not forth good fruit is hewn down, and cast into the fire.
Mat 7:20 Wherefore by their fruits ye shall know them.

Not everything that seems good is good. Paul wrote:
2 Cor 11:13 For such are false apostles, deceitful workers, transforming themselves into the apostles of Christ.
2 Cor 11:14 And no marvel; for Satan himself is transformed into an angel of light.
2 Cor 11:15 Therefore it is no great thing if his ministers also be transformed as the ministers of righteousness; whose end shall be according to their works.

God does not always tell us things we want to hear. Remember he is our Heavenly Father and fathers pass judgement and punishment. They tell us things we dont want to hear and things we supposedly already know. Peter wrote some great words of wisdom:

1 Pet 5:8 Be sober, be vigilant; because your adversary the devil, as a roaring lion, walketh about, seeking whom he may devour.

The Lord said in his word that nothing is new under the sun. All the tricks Satan seems to come up with are the same old thing in a new package.
Eccl 1:9 The thing that hath been, it is that which shall be; and that which is done is that which shall be done: and there is no new thing under the sun.

Be careful of all things. We get used to sin and our hearts become hardened. Some will call us fanatics, but we must point out sin and love and seek to please God more than ourselves and other people.
Mark 14:38 Watch ye and pray, lest ye enter into temptation. The spirit truly is ready, but the flesh is weak.