Ja ja, ich verstehe, was du meinst. Ich möchte gern auch so viele sprache wie möglich sprechen lernen.
Swedish is our second official language alongside Finnish, so everyone here has to learn the basics of it.
För att prova att jag ljuger inte skriver jag det här uttrycket på svenska. = Todistaakseni, etten valehtele, kirjoitan tämän lauseen ruotsiksi. = To prove that I'm not lying I'm going to write this sentence in Swedish.
And I think that there isn't anything particularly wrong in assuming stuff. However, you must understand the limits of the things it's safe to assume or else you'll ultimately end up basing your opinions and views on the foundations formed on your assumptions that are more than likely to be only partly true or maybe even completely false.
Here's an example: I see a man in a shop looking at the woman's clothes. I can safely assume that he's interested in bying a piece a woman clothing since, well, he's looking at them in a shop. However, I can't make any further assumptions about the person these clothes are for or the reasons he's bying them. Otherwise I'd also start assuming other things, like his personality and intentions. After this I'd automatically base my opinions about the man on these guesses and always see him from that perspective. If my original assumptions would've been even slightly off, it could end up giving me the completely wrong picture and at the worst case scenario lead me into hating the guy without any reason.
Similarly, it's easy to make false assumptions about people based on other things. These assumptions are rarely right and often the errors in them are quickly realized, even by the one who made them. Despite this they'll still stay in the back of the head and always affect the way one sees others. They also usually have a habit of highlighting certain things, be they good or bad, and creating overexaggerated and simplified images of others, leading into unjustified idolizing or hatred.
That was extremely far-fetched example and out-of-context explanation, but I hope you understood what I was going for with it. Basically, it's okay to assume some basic things. That just makes life easier for everybody since every little thing doesn't have to be explained. However, one musn't think he can judge a person by appearances/rumors/single acts/other stuff only, nor should he ever only trust assumptions. That's what I think.
Wall of text... and four different languages in one message!