It is G‑d Who causes the means of livelihood to be available to the person, and He is the one Who causes the person to be successful when engaging in those means. The author will now illustrate the folly of a person who places importance on his means, thinking that excessive toil or the use of unethical means will help him earn a living, when in truth he would have earned exactly what he was supposed to in an easier manner.

This man is similar to a person traveling in the desert who becomes dehydrated and finds bitter water in a well, and he greatly rejoices at finding the water and quenches his thirst with it. However, as he journeys a little further, he finds a spring that is flowing with sweet water. He is distressed that he previously drank the bitter water and quenched his thirst with it.

וְכַמָּה הוּא דוֹמֶה בָּזֶה לְאָדָם שֶׁהוּא בַּמִּדְבָּר הִכְבִּיד עָלָיו הַצָּמָא, וּמָצָא מַיִם שֶׁאֵינָם מְתוּקִים בְּבוֹר אֶחָד וְשָׂמַח בָּהֶם שִׂמְחָה גְדוֹלָה וְרָוָה מֵהֶם. וְכַאֲשֶׁר הָלַךְ מְעַט וּמָצָא מַעְיָן נוֹבֵעַ מַיִם מְתוּקִים, דָּאַג עַל מַה שֶּׁקָּדַם מִשְּׁתוֹתוֹ הַמַּיִם הָרִאשׁוֹנִים וּרְווֹתוֹ מֵהֶם.

If someone he trusts had told him that he would later find sweet water, he would certainly not have put himself through the pain of drinking the bitter water.

Similarly, the wealthy person whose wealth reached him through a particular means that involved much toil, were he to have stopped himself from engaging in that particular means, he would have managed to obtain his wealth through different, easier means —as explained earlier.

וְכֵן בַּעַל הַמָּמוֹן שֶׁהִגִּיעַ אֵלָיו בְּסִבָּה יְדוּעָה, אִלּוּ הָיְתָה נִמְנַעַת הַסִּבָּה הַהִיא מִמֶּנּוּ הָיָה מַשִּׂיג אוֹתוֹ בְּסִבָּה אַחֶרֶת, כַּאֲשֶׁר הִקְדַּמְנוּ,

The difficult work that he engaged in to obtain his wealth is the equivalent of the bitter waters in the parable. Were he to have realized that he could become wealthy through a different, easier means of obtaining wealth—the sweet water in the parable—he would never have engaged in all that hard work.
The author continues by quoting a verse that supports this idea:

Through a different, easier means. Without much effort or difficulty ( Tov Halevanon ) and in a manner that would be ethical and aligned with G‑d’s commandments ( Pat Lechem ).

Wealthy through a different, easier means, etc. This thirsty man had no faith that he would find fresh water, so he understandably drank the bitter water, but if he were guaranteed fine, fresh water, a wise person would wait for that. A person who wishes for sustenance and wealth must be certain that it all comes from G‑d, and G‑d will find an appropriate way for him to earn what he is supposed to. Therefore, he is not to employ unethical behavior or even be “invested” in the “vessel”; rather, he must know that he has “fresh water”—a Divine path through which his sustenance will arrive ( Marpei Lanefesh ).

Bottom line
G‑d is the true source of your success. There is no reason to choose a difficult occupation; G‑d can provide you with wealth through an easier avenue as well.

As it says (I Samuel 14:6): For with the L-rd there is no limitation to save with many or with few.

וּכְמוֹ שֶׁאָמַר "כִּי אֵין לַה' מַעְצוֹר לְהוֹשִׁיעַ בְּרַב אוֹ בִמְעָט" (שמואל א' יד, ו).

Since it is G‑d Who is the true source of his success, there is no reason for him to specifically choose a difficult occupation, thinking that it will cause him to be more successful. G‑d, Who is unlimited, can provide a person with wealth through an easier avenue as well.