'Serendipity' is sorta a complicated word in the sense it can be used across so many contexts, albeit in a hugely positive way. For one, we will never expect accidents/mistakes to be fortunate (statistically), but this word is the exact opposite of that. Two, this word epitomizes hope in its very meaning, and there is a transcendental connotation to it as well. Three, it is the kind of phenomenon we often see happening in romantic comedies, but we would truly like to see happening in our lives.
The meaning of the word is negative in the first place, but the fact that it's overused and viral makes it worse. This happens to a lot of other words as well, when a word can be used so much the meaning can be degraded or applied in a wrong way.