Zech. 1:13,17 And the LORD answered the angel that talked with me with good words and comfortable words....the LORD shall yet comfort Zion, and shall yet choose Jerusalem.

Life in this world throws a lot of discouraging things at people. Those returned exiles were discouraged and disheartened. They did have a desire to know and worship God, but conditions seemed to work against them. That's how it is with many of us today.
What they needed most at this particular time were kind words of comfort and encouragement.

And God gave them good and comforting words.

Isaiah 40:1 Comfort ye, comfort ye my people, say your God.
40:2 Speak ye comfortably to Jerusalem...her iniquity is pardoned:
40:28 Have you not known? have you not heard, the everlasting God, the LORD, the Creator of the ends of the earth, faints not, neither is weary? there is no searching of his understanding.
40:29 He gives power to the faint; and to [them that have] no might he increases strength.
40:30 Even the youths shall faint and be weary, and the young men shall utterly fall:
40:31 But they that wait upon the LORD shall renew their strength; they shall mount up with wings as eagles; they shall run, and not be weary; and they shall walk, and not faint.

A Jealous God?

Zechariah 1:14 So the angel that communed with me said unto me, Cry thou, saying, This says the LORD of hosts; I am jealous for Jerusalem and for Zion with a great jealousy.
Zechariah 8:2 This says the LORD of hosts; I was jealous for Zion with great jealousy, and I was jealous for her with great fury.

Compare with
2 Cor 11:2 For I am jealous over you with godly jealousy: for I have espoused you to one husband, that I may present [you as] a chaste virgin to Christ.
Deut. 5:9 Thou shalt not bow down thyself unto them, nor serve them: for I the LORD thy God [am] a jealous God

We tend to understand the word "jealous" as a sinful, selfish passion.
But God is not in any way connected to sinful passions. His awesome love is the very opposite to selfishness, so what is meant by these verses?
It is a strong emotional word and denotes strong emotions.

It needs to be understood in the light of God's great love for us, so great that
"God so loved the world that He gave (not just sent but gave) His only begotten Son that Whosoever believes in Him should not perish but have everlasting life, God didn't give His Son to condemn the world, but that the world through him might be saved.
He that believes on him is not condemned: but he that believes not is condemned already, because he has not believed in the name of the only begotten Son of God."

All through the gospels (especially in the book of John) we realize that Jesus is the life, the hope, the peace. He is everything we need to have hope and life. We see Jesus pleading, and weeping, saying "how can I give you up", "how often would I have gathered thy children together, even as a hen gathers her chickens under her wings, and ye would not! " and now you face desolation. " If you had known, even you, at least in this thy day, the things [which belong] unto thy peace! but now they are hid from thine eyes." (Matt. 23:37, Luke 19:42, Hosea 11:8)

God's jealous is due to His great love, His great desire to save and restore and give life, peace and happiness. He knows that apart from him, we have only trouble, sorrow and eternal death.

His jealousy reveals the intensity of His love toward His erring people. He is not going to be indifferent, He will be personally involved in winning us back, offering salvation, even giving His life to redeem us, calling us to turn to Him and find the hope, life, love and peace only He can give. He is not going to sit back and simply let sin devour us.
Yet, in all that, He will not force us to come to Him, our rejection of Him causes Him to weep. He has no pleasure whatsoever in the destruction of the wicked. He would save them if only they were willing.
God hates sin, for it destroys His beloved people, and their clinging to sin causes them to reject life in Him, and only leads them into misery and desolation.
Someday, sin will be destroyed forever.