Basically, as u/NoctiferPrime says, there is always something to do. On the second leg, there's always something I want to do. The key point is variation.
If you start from the ground up, humans are pattern recognition machines. Grind (in my experience) is repeating a pattern when you know it's a pattern and you feel forced to do it: It wears you down, because you know exactly what your future holds. Take farming Twilight texts in (Vanilla) WoW for example: There was no variation: Three camps, three kinds of mobs, and you apply your abilities (which also don't change) in the one most efficient way possible. Drop tables on those mobs also were very limited, so you had a grasp on what your gaming future would hold, down to easily eyeballing how many Twilight Texts you'd need to be "done".
Contrast Warframe: There still are patterns, but they are much more intermixed, and the player has much more control over mixing it up themselves. Sure, there are bosses that drop frame prints - but compared to WoW, at least the way to them is not always the same. And since you can change your entire play style by mixing up your loadout, you have some control over mixing the experience up a bit.
And if you're bored after running a boss ten times, there's a lot of other activities that offer benefits to you. There's almost always an alert or an invasion going on, and those alerts offer a wide variety of things you you might not be actively aiming for, but offer an incentive to not get bogged down in the sameness of the grind. These "benefits" actually showcase one more thing of variety: The variety of goals and the breaking up of linearity. On a side note: Those constant special events help immensely in the world/universe feeling alive, just as the "interactive" ship/menu does.
In most other games, you set your eyes on one thing, and you do that one thing until you have met your goal. You grind rep in WoW until you can buy what you want from the vendor, then you never visit him or the faction again if you can help it. You also don't "get" anything out of that linear investment of time into rep than "scrap" and the one thing you aimed for. In Warframe, you don't usually have one single goal, because even if you put in 20 hours farming for an Ash Prime, there's a lot of "side benefits" that you got from that activity: Mods you use, Tower Keys, rare resources, maybe you levelled up some gear, or other Prime parts that open up new goals for you to achieve, or alternate venues of attaining your current goal, through trading.
To put it in visual terms: There is linear grind, like in WoW. It's one string of twine after another, and they're mostly entirely separated from another: While you're farming Twilight rep, you won't further any other goals you have any, nor will you open up new things for you to achieve. Each string is for itself, in "stasis". This is the one extreme of the spectrum of patterning - a very easy-to-read, very mind-dulling one. The other extreme would be "patternless", or at least with a pattern you can do very little to influence, very random, like in Diablo III, where you can't even reasonably "aim" for a self-set goal. It's like a haystack where you just hop around in and hope that RNGesus smiles on you.
Warframe takes a third option: You have "leading" strings that you can work on to further a specific goal, but weaves randomization and alternate offers to aim for, strengthens the weave with "substance" (most activities don't exclusively benefit one single goal), breaks up repeating patterns with special events and thus creates a weave that strings players along for a long time.
Last but not least, it's freely accessible at any time (so you never feel pressured to "make the most of the time you paid for"), and most anything in-game can be achieved without paying for it, which creates a base feel of a level and fair environment to players.