Okay. This is definitely
a boundary question, and I have to say that my advice falls somewhere in the middle.
When you are still pre-2-years-or-so, you probably shouldn't be investing time and effort into anyone outside the M. That would be like trying to build a deck off of a house supported on toothpicks. The M is the most important thing, and your investment of energy should reflect that priority.
Even after the 2-years-or-so, your primary focus should still be on your S and your M, but (depending on how the temperature of the water is) you could theoretically start building on new friendships with others, with your S's blessing (no pun intended).
To salty_lt2's point, when you do get to this step, the "compassion" should be very appropriately framed. You need strong boundaries, and you're the only one who can police them.
Another analogy: Think of your boundaries as a neighborhood, and you are the neighborhood watch. If you do a really crummy job of keeping the neighborhood safe, no one is going to trust the neighborhood or the neighborhood watch and no one will want to live there with you. On the other hand, if you do a very good job of keeping the boundaries, your S will notice, and your S will decide that it is safe to move back in.