Will u begin to guard if someone is friendly to u all of a sudden?

yes, certainly. but i won't show it too much. usually their motive are to make us do something for them.

Well as a result of watching too much Crime Investigation channel, I am now conviced that you cannot really trustI strangers. Most often than not the ones who seem to be nicer and friendlier strangers are the ones who is really a criminal. I live in the city and have been victimized twice by a hold up so these days I don't really talk to anyone from the street, even those asking for directions because it just might be their ploy. Simply put I don't trust strangers at all.

i'll. i'll guess what he/she really wants from me. one of my classmates, who has broken friendship with me, suddenly called me, being very friendly. i was confused by her and felt awkward. with day passed by, finally i got it. she didn't want to recover the relationship. she just wanted me to help her learn English to pass the CET-4. that's human nature, no one will be good to you without any reason. well, i don't mean it in the bad aspect. someone will be very very good to you just because he loves you. haha.

Not that much. Except if it's really that suspicious. Well, anyways, I haven't met that experience yet. Maybe my mortal enemy, (whoever she/he is because I don't have any), who suddenly seems friendly, I will really guard myself.

Not really. But I would not be that naive to the relation between us. Nevertheless, being extra careful is not bad too.