The 'main motivation' (or "goal") of No Man's Sky is to reach the center of the universe, as stated many times in various interviews and [written / video] previews. You don't have to explore all the time, or you don't have to only do that exclusively while doing nothing else. If you want, by choice, to reach that center of the universe, then your immediate goals should be simple and clear. You'll gather resources by various means, upgrade your ship and suit (mostly your ship), and you'll then be able to reach more star systems and eventually leave the galaxy you started in (it was confirmed that there's multiple galaxies, it's not just one big ball with a center in it). And from that point you'll probably have a better idea as to where you need to go next, another galaxy perhaps, and if not you might be able to start your journey toward the center right away.
If, however, you don't want to do that, then your motivation(s) are merely the ones that you give yourself. If you don't want to, nor can "guide" yourself, if you cannot motive yourself to do what can be done in the game, then of course it won't be a game for you just as much as a game like Fallout 4 or Skyrim wouldn't be (in those games there's a main goal too, and you can avoid or ignore most of the side content and focus on the "main goal" if you wish to, finish the story eventually, look at the credits screen and then call it a night). Essentially, it's up to you, the player. No Man's Sky isn't there to spoon feed us goals, WE create them based on what CAN be done in the game (trading, fighting, gathering, upgrading, exploring, discovering; pick your favorites and do those things).