measure of compatibility

hurmmm... how do u measure whether u are compatible with other ppl... ur frens, ur loved ones.. ever seen a pair of two frens that are totally different, like u cant even think of these two guys or girls can be best buddies.. but yet they are.. totally click.. same goes with two lovers that definitely catch ppl's attention... a beauty and a beast perhaps...yea,sumtimes ppl said that u need the syncronisation to click,the same hobbies, same interest, same attitude... but somehow i feel the opposite thing..