Repeated exposure to words, phrases or objects gives rise to an unconscious association of happiness to them. In terms of evolution, this is just the habituation to something we routinely see that poses no risk. In fact, not only do we get used to them, we actually associate happiness to these objects.
An interesting study was done in a student newspaper, where random turkish words were printed in a box on the front page. Some of the words were repeated numerous times, whilst others just once – this was done at several universities, with different numbers of multiplicity for each word (i.e. in one newspaper, a word was printed numerous times over the course of the weeks, whilst that word in another newspaper was printed only once – and vice versa).
This practice was continued for several weeks at different universities until a study was taken, seemingly (to all participants) unrelated to the newspaper boxes. When asked whether each turkish word had a positive or negative sound to it, a significant number of students associated the turkish words which were repeated more with a more positive response compared to the less often repeated words.
[edit: this study was mentioned in Thinking, Fast and Slow by Daniel Kanehman — though he didn’t cite the paper… since he’s a Nobel Laureate, I imagine citing him is enough!]
This is probably one (of the many) reasons why Coca-Cola and Pepsi still advertise even if the entire world is likely to have heard of them.