When you want to convey something of great importance, whether in a workplace environment, or teaching your children, having that person repeat your instructions back to you helps to ensure the accuracy of your message. Psalm 136 uses the same method to teach us a foundational truth about God.
Steadfast. Love. These are strong, solid words that communicate their meaning. For instance, steadfast: faithful, always present, never alone, will not leave, always listening, always paying attention, working on our behalf, forever.
And love: what comes to your mind when you hear this word? For some of us our past can radically color the meaning of this word but the accepted definition is, "an intense feeling of deep affection." Other descriptions of the word include, passion, tenderness, warmth, intimacy, delight, taking pride in, safety, always present, selfless, always seeking the best, and so many other descriptions.
But the fact remains that Creator God is telling us that He loves us, repeatedly, (26 times just in this Psalm) as He seeks to plant this truth deep into our soul.