Uke (literally "one who receives", the one who takes the fall) and nage (the thrower) have a very special relationship. Unlike many martial artists who train against an opponent, the aikidoka trains with a partner. There is no competition in aikido, no pitting of one person against another. Instead, each partner is half of a whole, each having equal responsibility for the learning experience.
Contrary to what one might think, uke --not nage -- has the most difficult role. Uke has the task of giving his partner an "honest" attack to work with. On the face of it, this seems quite simple. Actually, it is not. An honest attack is more than holding as tightly as you can, or striking as forcefully as you can. An honest attack is an aware attack. Aware of your partner's situation. Is there a major difference between partners in size and strength? In experience? Obviously, if a 200 pound black belt holder with advanced skills were to strike full power at a tiny beginner with little or no experience, he or she would be more than just intimidating, and in fact would be totally insensitive and irresponsible.
Many students of aikido have a difficult time in reconciling the difference between total resistance/full power attack and "falling down" for their partner in a condescending response to a weakly-applied technique. It is helpful to remember that as uke, your primary responsibility is to serve your partner. Try to bring out his or her best. This is best accomplished through sincerity and sensitivity: by bringing them to the edge of their capabilities and extracting their maximum performance, but without undue strain.
The relationship between uke and nage is like a cart with wheels. Tighten the hubs of the wheels too much and the cart will not roll. Too loosely and the wheels will wobble and provide no stability. Think of the movement of your techniques as the movement of the cart: tighten the hubs of the wheels just to the point where they begin to bind, then back off slightly and the wheel (technique) will run smoothly. Over a period of time, consistent repetition of correct movement firmly executed will lead to naturally stronger technique. Remember: train, don't strain!!!