You bought a cheap bottle of wine, but you actually liked it. You are telling your friend about this wine. You say:
I was pleasantly surprised.
You can be "surprised" in both positive and negative ways. When you are surprised in a positive way, you can say that you were "pleasantly surprised".
The word "pleasant" means "good". So why do people say "pleasantly surprised" instead of "positively surprised" or "surprised in a good way"? This is an example of collocation: these words are just paired with each other.
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