In Parashat Vayetzeh, Yaakov goes to Lavan's house, but prior to his arrival he meets up with Rachel. Shockingly, the Pasuk says that he kissed her and cried (29:11). How exactly are we to understand this episode? Do we not say that the Avot kept the whole Torah before it was given, and can we not assume that the righteous Yaakov would not do something so immodest as to kiss a girl he isn't married to yet - in public?
The gemaroh in Kesubos daf 17 says that Rav Acha danced with the kallah on his shoulder. When the Chochomim asked if they were allowed to also? He answered - if she is like a beam in your eyes then you could do it too. But if not then certainly not. That was all only in those days when they were on a tremendously higher level then us today. Nowadays, shulchan aruch (pischei tshuva on Even Haezer 65:1) rules that we are not allowed to because nobody could say that holding the bride effects him like a wooden beam.
Many commentaries have offered explanations to this issue. Even if you disregard societal differences of modesty or niddah concerns, there are other possibilities. Here are a few:
Rachel was too young to arouse passion
The Seforno indicates (on 29:18) that Jacob had to work so long for Lavan so that she be old enough to marry:
Jacob was related to her and old.
This argument bolsters the first and is usually explained concurrently. As the Seforno comments, this is why in the verse after the kiss does Jacob tell Rachel that they're related :
In this case, being related and the age gap permitted it in the spirit of the halacha (Shulchan Aruch Even HaEzer 21:7) that parents can kiss their young children.
Jacob kissed her head/shoulder/hand or blew her a kiss
Ramban offers this other explanation. He refers to Ibn Ezra's commentary on Jacob kissing Isaac (27:27). In both cases, the verb "kiss" is following by the preposition "to" or "towards." This could mean that he didn't actually kiss her, or that it wasn't on the lips. Ramban writes:
It seems that explanation softens the potentially erotic conjuring of kissing.
There are other explanations, especially kabbalistic ones, but these are the most common peshat explanations.