Why isn't this code working without an else block to the if statement? - Python

First post in here :)

So what I'm trying to achieve is, if a character is found inside a list in 'key' list then add the 'other' character in that list to the variable 's'. If a character is not found inside a list, then add the character itself to the variable 's'.

message="hello !"
s=""
found=False
key=[['m', 'c'], ['u', 'e'], ['b', 'g'], ['a', 'k'], ['s', 'v'], ['h', 'x'],['i', 'z'], ['r', 'y'], ['p', 'w'], ['l', 'n'], ['o', 'j'], ['t', 'f'], ['q', 'd']]
for i in message:
    for j in key:
        if i==j[0]:
            found=True
            s+=j[1]
            break
        elif i==j[1]:
            found=True
            s+=j[0]
            break
    if not found:
        s+=i
return s

input:

hello !

expected output:

xunnj !

Somehow the later part(add the character itself to the 's' variable) does not work unless I put an else block to the first if statement. like this,

if i==j[0]:
        found=True
        s+=j[1]
        break
elif i==j[1]:
        found=True
        s+=j[0]
        break
else:
        found=False

output without the else block:

xunnj

I want to know why,

if not found:
        s+=i

does not work without an else block to the first if statement. Thanks.