In this case, the answer to both is pretty similar, with some variation. The US and Israel fear their hegemony being threatened. At the moment, especially in the context of the MiddleEast, the main threat comes from the Islamic Republic of Iran, and its influence across the region. The Islamic Republic has a solid relationship with the Iraqi leadership, and more importantly, a deep connexion with a large portion of the Iraqi population. The Islamic Republic has friends within the populations of Bahrain, Yemen, Lebanon, and to King Abdullah’s eternal horror, Saudi Arabia. The Islamic Republic has an alliance with Syria, a profound brotherhood with Hezbollah, and a at best good, at worst decent relationship with the Palestinian resistance. This has been termed by Iran and Hezbollah as the Axis of Resistance, i.e. a cloud of power that resists foreign hegemony, with Israel perceived as a foreign entity occupying Palestinian land.
The second question was, what role does Assad play here? The Islamic Republic and Hezbollah have been quite clear that Assad has bridged a gap for the Resistance, a bridge which hasn’t only irked Israel, it has threatened it. Assad, viewed alone, may not be a threat to Israel, but viewed as the middleman, as the bridge, as the road from Tehran to Bint Jbeil, is a dangerous individual. This is why US-Israeli interests require Assad to leave.
Many of the Tamarrod Protestors in Egypt did not want military rule, but they became unwitting pawns to much higher powers with their grander designs. I do not respect pawns, however noble their intentions. The rebels in Syria are pawns (and their intentions are, by and large, far from noble.) One should see through the illusion.