Tafsir of Ash-Shu`ara' 26:135

(Indeed, I fear for you) is in the position of a causal explanation; meaning: I fear for you—if you do not fear [Allah] and offer gratitude for these blessings—(the punishment of a Great Day) in this world and the Hereafter. For the ingratitude of a blessing entails punishment, just as being grateful for it necessitates its increase. Allah the Exalted said: "If you are grateful, I will surely increase you [in favor]; but if you deny, indeed, My punishment is severe."

The explanation was provided in this manner, rather than mentioning that fearing [Allah] leads to an increase [in blessings], because the loss of a blessing causes more grief than its increase causes joy, and repelling harm takes precedence over attracting benefits.