It is a Person’s Nature to Take Credit for Himself

פֶּן תּאֹכַל וְשָׂבָעְתָּ אּוּבָתִּים טוֹבִים תִּבְנֶה וגו', וְרָם לְבָבֶךָ וְשָׁכַחְתָּ אֶת ה' אֱלֹקֶיךָ וגו', וְאָמַרְתָּ בִּלְבָבָךָ כֹּחִי וְעֹצֶם יָדִי עָשָׂה לִי אֶת הַחַיִל וגו' (ח, יב-יז)

Lest you eat and are satisfied and build good houses… And your heart will be uplifted and you will forget Hashem, your G-d… And you will say in your heart, “My strength and the might of my hand did… this wealth.” (8:12-17)

The Alter of Kelm zt”l would say that the pasuk does not say that one might say these words. Rather, it says that one “will say” them. This is because it is the nature of a person to try to take credit for his successes due to his own talents and strength.