Look there at that second word in your Greek quote of scripture: γάρ; this small word in Greek, a particle, which according to BDAG:
...is a conjunction used to express cause, clarification, or inference.
(This is key to answering your question.)
In BDAG under 1.) it is stated: marker of cause or reason
Under 2.) it states: marker of clarification, 'for', 'you see'
In Zodhiates' CWSDNT he states: γάρ GAR, a causative particle expressing the reason for what has been before, affirmed or implied. For, in the sense of because, and so forth.
Under D.) Put by way of explanation or demonstration:(1)Where it merely takes up a preceding annunciation and continues or explains it, like the English "namely,""to with,""that is,"
Read Matthew 11:25-12:8; there, is what has gone before. There, you will find the presentation of God (who is the Father) being mingled in and with Jesus Christ, becoming the way of knowing God and of having the possibility of being supplied with the real rest; the rest that is the Sabbath.
γάρ...Lord over what is the Sabbath is the Son of Man.
Because He, has (11:27) had delivered to Him, of the Father, all things. And no one knows the Son but the Father; and no one knows the Father but the Son and he to whom the Son will reveal Him.
...and 12:7, But if you had known what this means: "I will have mercy and not sacrifice, you would not have condemned the guiltless!"
γάρ...(it is in light of all this that) Lord, over what is the Sabbath, is The Son of Man.
It is as Hebrews 1 says...God had spoken in the past to the fathers through the prophets in many different ways, but has in the last of these days spoken to us...
In Son... that is, Jesus Christ.
So we can understand, γάρ, that even in regard to the Sabbath, Jesus Christ is The Way, The Truth, and The Life.