Users have a desire to seek out missing information.
<aside> ℹ️ The curiosity gap (also known as The Knowledge Gap) is the space between what users know and what they want or need to know. Gaps cause pain, and to take it away, users need to fill the gap.
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Graphic: Built for Mars
For example, LinkedIn used to show you a graph of your profile views that was greyed out with a dialog box prompting you to get a premium account to access this data. Linkedin’s design was leveraging user's curiosity quite effectively by showing them this graph was available and within their reach, they just needed to upgrade their account to get the graph, fill their information gap and satisfy their curiosity.
UX Psychology: 🚧 Knowledge Gap