Since aikido is a defensive art, it's important to have something to defend against in the dojo. Half-hearted attacks don't cut it. No follow-through, no aikido. If someone attacks like a kindergartner on the street, it's easy to avoid. No defense needed.
So in class, uke (attacker) needs to give tori (defender) a committed attack. Not trying to rip his head off, mind you, but more than a feeble push or a girly-man wrist grab. I was gracefully introduced this simple fact during the last class.
Specifically, you have to launch the attack and also "keep in contact" so your momentum will enable tori to complete a technique. This doesn't mean just pushing with an arm, but one's whole body should stay with tori, like in a real attack. I found this is especially important at my beginner's level, where all attack and defense is done more or less in slow motion. If there's no presence, no weight and no follow-up to an attack, tori cannot do aikido. I did not fully realize that my attacks were about as menacing as wet spaghetti.
Rather than being too slack or stiff as a board, staying with tori actually makes the technique both easier and safer. I was able to take ukemi (forward and backward rolls) better after a more pro-active attack.
This wasn't something I learned on my own. Sensei and other partners formally showed me how to do this. Firmly. Still don't have it down perfectly, but at least I know in what direction I'm heading, so to speak.