The sentence contrasts 'weakness' with 'strong and ..........'. When someone else appears weak, it can make you feel more confident, self-assured, and willing to assert yourself. 'Assertive' means confident, self-assured, and inclined to express oneself boldly - which completes this psychological contrast perfectly. 'Attentive' means observant, 'avid' means enthusiastic, and 'averse' means opposed - none of which create the same contrast with weakness.