There are certain scenarios when autofocus does not work or determines that there is not enough light to properly focus. These are described in your manual (geometric patterns, subject behind objects (e.g. fence), etc.)
Also, certain focus points are only sensitive to one direction, and others (cross type) are sensitive to both directions. If you are using a non-cross type autofocus point, autofocus also can decide that it does not have enough information, and use your focus assist light. More info on focus point types.
To test for the latter, when you see your focus assist light is being used when you do not expect that, try switching to single capture (S) mode, focus with your center focus point, which is a cross-type focus point, and also gets the most light, being on the optical axis of your lens, and try to see if focus assist is being used again, that time.
Aside of the considerations above, it might happen that your focus sensors got some dust or dirt, and not getting enough light.