Extreme ideas and opinions are commonly seen growing in many aspects of today's society, ranging from political ideology to healthcare choices and from dietary preferences to technological innovation. Such a trend may be understood as an outcome of a spontaneous dynamical process driven by the recent advancement of information communication technology that allows people to preferentially select their information sources. Here we study, using an agent-based model of adaptive social network dynamics, how extreme ideas may arise in society in which individuals simply try to conform to social norm within their social neighborhood. Our model assumes that each node gradually assimilates its state to local social norm, i.e., the average of its in-neighbors’ states, while also changing edge weights based on their states. Numerical simulations revealed that, when individuals tend to practice homophily by strengthening their ties selectively to neighbors with similar states, there tends to be many extreme ideas emerging in society while the network topology tends to become fragmented. Such outcomes are mitigated, however, when individuals also practice novelty-seeking behavior by increasing attention to neighbors whose ideas do not conform to the local social norm. These results paint a paradoxical picture of complex social processes — society produces difference when individuals seek sameness, or society reaches sameness when individuals seek difference.