Why use 居然 or 竟然 in this sentence?
In my latest notebook I wrote a line 我猜我很笨到再相信你, by which I meant I guess I was stupid to trust you again.

Now I can understand people will correct it if its wrong but I completely dont understand why people are putting 居然 and 竟然 into it.

I have seen 我猜我很笨，居然會再相信你, 我觉得我太笨了居然又相信你了 and 我觉得我很笨竟然会再次相信你...What has 居然 and 竟然 got to do with it, I thought these words means unexpectedly or surprisingly?ok, thanks for the replies so far, but now I'm even more confused :( I know that 居然 and 竟然 show surprise, but there is no talk of surprise or even an implied meaning of surprise in my sentence 'I guess I was stupid to trust you again', in fact, if anything it is the opposite! Please could someone try to explain this in English, I really don't get it.

Question

Why use 居然 or 竟然 in this sentence?
In my latest notebook I wrote a line 我猜我很笨到再相信你, by which I meant I guess I was stupid to trust you again.

Now I can understand people will correct it if its wrong but I completely dont understand why people are putting  居然 and 竟然 into it.

I have seen 我猜我很笨，居然會再相信你, 我觉得我太笨了居然又相信你了 and 我觉得我很笨竟然会再次相信你...What has 居然 and 竟然 got to do with it, I thought these words means unexpectedly or surprisingly?ok, thanks for the replies so far, but now I'm even more confused :( I know that 居然 and 竟然 show surprise, but there is no talk of surprise or even an implied meaning of surprise in my sentence 'I guess I was stupid to trust you again', in fact, if anything it is the opposite! Please could someone try to explain this in English, I really don't get it.

Answer

yes, when sth 居然/竟然 happens, it happens unexpectedly/surprisingly.

这么好的天居然下雨了。
I can't believe it's raining in such a nice day.

here if you say "居然又相信你了", or "i can't believe i trusted you again", then clearly it's not about surprise, it just shows that you're angry and regret trusting him.

you can't translate "i guess" literally as "我猜", i think "猜" in chinese is used only when you're actually making a guess. maybe we can just eliminate it in the translation.

I guess I was stupid to trust you again. 
是我太蠢了，再次相信了你。(蠢 is better than 笨 here)

and i think it's not wrong to use 居然/竟然, it's just more direct and strong.
我真是太蠢了，竟然又一次相信了你。
我真是个笨蛋，竟然又上了你的当。

Answer

in fact,chinese people don't say"i guess i'm ........",(just like 我猜我很笨”）instead，we often say i'm .......(just like 我很笨）this is the a difference between english and chinese,as to 竟然/居然，they are the same meaning,and in the sentence,people use"竟然",maybe he just want to show that how foolish he was to trust that person.

Answer

居然和竟然是一样的，他们是同义词（they are synonyms，you can use either of them!）

Answer

I saw your question about why we put 竟然 and 居然 into the sentence. 竟然 and 居然 are adverbs to be used to modify verb. They make  tone more strong. 
Compare:
I am  stupid to trust you again. 
I  am so stupid that trust you again.
The effects of 竟然 and 居然 just like that of "so".

Answer

Damn I wish people would not delete answers after they write them. To the girl that has deleted her answer..why? Im sorry people gave it so many thumbs down because it was wrong, but I did try to be nice and offer to try and explain this saying to you further. I was also able to see a very useful saying from you anyway, I had not had the chance to write it down though and now you have deleted it :( There is no shame in getting things wrong and you should not take peoples markings though the silly thumbs up, thumbs down system on here to heart. They means nothing!