Why did the Israelites complain at Marah?

The complaint of the Israelites at Marah stemmed from their immediate physical need for water after days without it. Despite having just witnessed God's mighty acts in delivering them from Egypt and parting the Red Sea, they forgot His faithfulness and provision. This serves as a reminder of the human tendency to forget past mercies in the face of new trials. God allowed them to face this test to teach them about their spiritual state, revealing their weaknesses and the need for a faithful reliance on Him. Such lapses in memory often lead to grappling with bitterness, yet they also provide an opportunity for growth and deeper faith.