Context
A warlock in the game that I run suffers from two levels of exhaustion:
Level 1: Disadvantage on ability checks
Level 2: Speed halved
This warlock knows the spell levitate. He is so tired—it's a hangover from a very potent homebrew drug, named Flashberries—that he levitates himself slightly to alleviate his body by hovering instead of walking. Yesterday, instead of walking/floating around by himself, he held onto the shoulders of his ranger ally and let the ranger "pull" him forward (as the ranger walked at normal speed).
Ruling
When the players came up with this idea, I ruled that this method of movement is possible. Because the warlock is afloat, the spell doesn't burden the ranger with extra carrying capacity while the warlock is not using his own movement, so both the ranger and warlock could now move at normal speed. The ranger effectively pulls forward a seemingly weightless person.
I thought it was quite the clever plan to avoid this part of his exhaustion, and I didn't want to slow down gameplay by looking up the rules, so I ruled in favor of the party.
Research & Question
Now that I'm reading up on both the spell description and What form of movement is granted by Levitate cast on yourself?, I realize we might've misunderstood how levitate works. Zooming in on the relevant parts (emphasis mine):
The target can move only by pushing or pulling against a fixed object or surface within reach (such as a wall or a ceiling), which allows it to move as if it were climbing.
and
If you are the target, you can move up or down as part of your move.
So this makes me think that the answer to my question would be No, because the ranger is not a fixed object or surface. And since spells only do what they say they do, I'm more inclined to think that, by RAW, the ranger is unable to move the warlock at all. It also looks like the warlock is also not able to actually hover horizontally by using this spell (oops).
Could anyone give a clear yes or no to my question, by RAW? And if either of my interpretations are incorrect, please explain why that is.
Best Answer
You already have the proper solution
A spell's text details the spell's effects — the only thing the spell does. Any additional effects are up to the DM. The DM role is crucial, because rules do not cover all the corner cases. This is a corner case, so you can not figure out the "correct" answer based on rules only, without the DM.
Good for you! Your player came with a creative solution and was rewarded. Unless he tries to use this movement method in combat, in terms of "balance" it really does not matter.
Jeremy Crawford, the lead game designer, suggests to choose what is "best for your story":
That is exactly what you've already done.