But why? Why does God only have mercy on some and not all? This makes me look at God differently for sure.
That's not even the point of these verses, verse 18 is:
Exodus 33:16-20 And how shall it be surely known, that both I and this people have found favour with Thee, except only if Thou go with us? So both I and thy people shall be glorified beyond all the nations, as many as are upon the earth. [17] And the Lord said to Moses, I will also do for thee this thing which thou hast spoken, for thou hast found grace before Me and I know thee above all.[18]
And Moses says, Manifest Thyself to me.[19] And God said, I will pass by before thee with My glory, and I will call by My name, the Lord, before thee; and I will have mercy on whom I will have mercy, and will have pity on whom I will have pity.[20] And God said, Thou shalt not be able to see My face; for no man shall see My face, and live.LXX
Moses asked God to manifest Himself, v18.
In the New Covenant, the Son will manifest Himself to those who have and keep His Commandments, v21... then both Father and Son will make their home with you.
John 14:21-24 He that hath My Commandments and keepeth them, he it is that loveth Me; and he that loveth Me shall be loved of My Father and I will love him, and will manifest myself to him.[22] Judas saith unto Him, not Iscariot, Lord, how is it that thou wilt manifest Thyself unto us, and not unto the world?[23] Ιησους answered and said unto him,
If a man love Me he will keep My words, and My Father will love him, and We will come unto him and make Our abode with him.[24] He that loveth Me not keepeth not My sayings; and the word which ye hear is not Mine, but the Father's which sent Me.
Think about that for a second. The Creator will live with you, if you keep His words. Deuteronomy 18:18-19 says the Father's words are the Son's words to His 12 Disciples, and so does John 17:6-8.
How is that not mercy? Mercy means He will forgive you when you repent. Your mercy towards others mean you forgive them when they say they're sorry. And if you love them, then you keep forgiving them, each and every time they say they're sorry.
It's not unconditional love. It's even better than unconditional love. Because when you hurt someone, you feel guilty. And you can't stop feeling guilty until you say you're sorry. But if you never had to say you're sorry... wouldn't you still feel bad about what you did?
Saying sorry clears the air, wipes the slate, and puts you back at square one.
And if you keep saying you're sorry, it changes you. You stop and think before you do something that makes you have to be humiliated. Because this is the real deal about the Sermon on the Mount in Matthew 5--Matthew 7: People with a lot of pride won't want any part of it.
But it's kinda hard to be proud, when you do bad things, right?
So, you either fess up, or you pretend it didn't happen.
And if you keep pretending stuff didn't happen, then you become so hard-hearted that you end up doing something really bad before you even notice what you have become.
Repentance and forgiveness... really IS mercy, in the Gospel of the Kingdom of Heaven.