Qitzur Shulchan Arukh – 189:2-3
If he unburdened and loaded [the animal] and it fell again, he is obligated to unburden and load it another time, even 100 times, as it says “you shall surely release it”, “you shall surely put it up with him”. [Literally, “release you shall release”, and “set up, you shall set up with him”; this lesson is being derived from the doubling of the language used to denote “surely”.]
Therefore, you must go with him up to a parsah [2.4-2.88 miles], because maybe he will need him, unless the person with the package to be carried says to him “I do not need you.”
The mitzvah of unloading [an animal] must be done [even] for free. However, to load [an animal], he is not obligated unless he is paid, and similarly for his traveling with him [see above], he must pay him. [Presumably because unloading the animal is an exercise in compassion for the animal, but reloading it is more a service for its owner.]
I am wondering how generalizable the obligation of “loading an animal” is. Does it mean there is a general duty to accept a job that would aid another?