It is unusual for a butterfly to land on a human,
as they are constantly moving around., and naturally quite timid insects. Humans also obviously don't provide any
benefits for a butterfly in the way that flowers do, therefore there
is no biological reason for them landing on a human. In fact, what happened to you is
pretty rare!
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Having a butterfly land on you is very symbolic though, and there are lots of different beliefs as to what this unusual event means:
Some people think that when a butterfly lands on
you, it is a sign of good luck, as it doesn't happen to many people. You have been picked out by the butterfly.
Other people believe that a butterfly can be a sign
from God. When the butterfly lands on you, this represents God
making contact with you for a reason.
Butterflies are also interpreted as a symbol of change coming your way. This is associated with the way a butterfly is the result of a metamorphosis from a caterpillar.
I'm not sure which of these I believe, but as all of the interpretations are positive, I think it is safe to assume that it is a positive thing that
this little creature landed on you.
However there are skeptics out there who would argue that a butterfly landing on you doesn't really mean anything. After all, a butterfly is a creature just like us - and so sometimes they need to rest. Your leg could have been the closest surface available for it to land on, so it was simply chance that it landed there for its rest stop.