勢不容己 There Is No Help For It
你進洞兩遭了，再進去一遭，管情救出師父來也。」行者揩了眼淚道：「也罷，到此地位，勢不容己，我還進去。你兩個沒了行李、馬匹耽心，卻好生把守洞口。」你进洞两遭了，再进去一遭，管情救出师父来也。」行者揩了眼泪道：「也罢，到此地位，势不容己，我还进去。你两个没了行李、马匹耽心，却好生把守洞口。」
逐句解 Line by line
You have been into the cave twice. Go in once more and you will be sure to bring the master out."
管情 is you may depend on it, an emphatic old promise-word. Bajie is very confident about a journey he is not making.
Pilgrim wiped his eyes and said, "Very well. Things have come to such a pass that there is no help for it. I will go in again.
也罷 takes 罷 at bà, the full fourth tone — the resigned so be it of a man agreeing to something he would rather not. 勢不容己 is the force of circumstances leaves no room for oneself: no choice in the matter. 還 is hái, once more, still.
You two have no baggage or horse to worry about now, so mind you guard the cave mouth properly."
A joke with an edge on it. Their duties have been simplified because they lost everything they were meant to be guarding, and Pilgrim makes sure they hear it. 好生 is properly, carefully — still ordinary in northern speech.
Pilgrim agrees, and the way he agrees matters: not with a plan, not with defiance, but with a phrase meaning there is nothing else to be done. He is going back down because there is no alternative, which is a different sort of courage from the one he usually gets to show.
His parting instruction is a wound dressed as an order. He has spent the last two passages in tears and has recovered enough to be sarcastic, which in this novel is how you can tell a character is going to be all right.