The quote advocates a positive response to negativity. The first part emphasizes smiling as a deliberate choice. It suggests a refusal to let haters affect one's mood.
The second part highlights the recipients of the smile: the haters. It implies a response of grace and composure in the face of criticism.
In essence, the quote promotes resilience in the face of criticism. It encourages responding to negativity with positivity, symbolized by a simple smile. It's a reminder to rise above and maintain one's own emotional well-being despite external negativity.