This is such a lyrical word - there's sheer joy embedded in the word. The very purity of feeling that this word conjures is enough to brighten the darkest moment...the best thing is that this word is that it is linked to the unexpected, and so, it describes feelings arising from unexpected incidents that range from the very, very ordinary to the very, very elegant...
Think - we very skilfully knit a cocoon around our lives - everything sooner or later falls into a pattern, knit and purl. We wiggle down and are very comfortable in our cocoons...suddenly, the heart tells us to do something not usually done - light a candle on the dining table, get a bottle of wine for the last dinner of the month (that's the most difficult meal to plan, often!), go for a walk, have a coffee and sinful dessert in one of those once-in-a-way-splurge places, get togged up just for the fun of it...anything...don't plan...the sheer serendipity fills the heart with radiance...
Maybe we should allow for more serendipitous moments to happen?