Do not submit: Most of the English translations are along the lines of "be not held" or "be not entangled." Each variation implies that we have a choice in the matter. Submission is an act of the will. We may, if we choose, accept the yoke of slavery, but it cannot be forced on us, since Christ Himself has set us free.
again: The yoke of slavery is known to all. Each of us has worn it. Many of us, I suspect, have submitted to it again and again throughout our lives.
Now, why would anyone do this, submit again to slavery? St. Paul goes on to suggest a reason:
For the flesh has desires against the Spirit, and the Spirit against the flesh; these are opposed to each other, so that you may not do what you want.
Not only may we prefer the usual desires of the flesh to those of the Spirit, but it's entirely possible for us to confuse the two, to -- if you'll pardon the overextended metaphor -- unhitch the yoke of slavery from the plow of carnal pleasures, say, only to hitch it to the plow of self-righteousness, without noticing that we're still yoked to slavery.
to the yoke of slavery: We must not think that to be free in Christ is to be free of all burdens, to be out from under all yokes. Jesus Himself promises us a yoke in on of the tenderest passages in Matthew:
"Come to me, all you who labor and are burdened, 16 and I will give you rest. Take my yoke upon you and learn from me, for I am meek and humble of heart; and you will find rest for your selves. For my yoke is easy, and my burden light."
And a few verses after St. Paul warns the Galatians against submitting again to the yoke of slavery, he orders them to serve each other:
For you were called for freedom, brothers and sisters. But do not use this freedom as an opportunity for the flesh; rather, serve one another through love.
In both cases, there is servitude: the old servitude is enforced by a yoke, the new servitude is given in love. Or, if you like, in both cases there is a yoke: the old yoke is of slavery, the new yoke is of love.
The human spirit chafes against its fallen nature, and the unwise interpret this as a sign that servitude of any sort is contrary to human perfection. The fool thinks he can only achieve freedom by refusing to serve, but those given the wisdom of the Holy Spirit know the freedom they were called for, the only true human freedom, is a freedom of love, and to love is to serve.