When you say that someone is lukewarm, it is usually not a compliment, it means you never know what to expect from them. Can you trust them? Should you be wary? In everyday life, dealing with such beings is complicated, because you cannot predict to which side they will lean.
Try this experiment: immerse your left hand in hot water for a moment and your right hand in cold water; each of your hands gradually reaches the temperature of the water in which it is immersed. Now place both hands in lukewarm water: you will see that your left hand finds it cold, while your right hand finds it hot.
It is therefore impossible to tell: is the tepid water hot or cold? Similarly, if you are satisfied with ‘lukewarm’ thoughts and feelings, you can never have a clear idea about anything, and everything you do is indecisive, wavering.