God is complete as He inform us in the book of Revelation - and in order to warn us and for our benefits not to go through the great tribulation.
About the seal: there are seals for the anti-Christ the mark of the beast and seals for the servant of God - all in all for the seals in the Holy Spirit are 144,000
1Then I looked, and there before me was the Lamb, standing on Mount Zion, and with him 144,000 who had his name and his Father's name written on their foreheads. 2And I heard a sound from heaven like the roar of rushing waters and like a loud peal of thunder. The sound I heard was like that of harpists playing their harps. 3And they sang a new song before the throne and before the four living creatures and the elders. No one could learn the song except the 144,000 who had been redeemed from the earth.
Romans 12:15 Rejoice with those who rejoice mourn with those who mourn.
Our sinful nature will always do the opposite of what the scripture says. Most of the time, we don't rejoice if others are rejoicing because we are jealous of their status in life. And if they are mourning we enjoyed seeing them suffering with their problems, we don't even try to comfort them. But as believers in Christ Jesus we need to obey His words; God said "love one another". Love does no harm to its neighbor but sympathizing them of what they feel. So, when other are happy, be happy with them. If they are sad, share with their sorrow. And verse 16 Live in harmony with each other.