Before we answer any question, we examine it to see if there are any inaccurate statements being made within that question and clear that up. First, it's not Islam that is aggressive, and it's not all Muslims that are aggressive either (for just one example, see the Muslim community in England's reaction to three murdered young men during the horrific London riots of last year).
Your question can be split up into two components:
- Why are so many Muslims so passionate about their faith?
- Why do so many Muslims use bad behavior when provoked?
I'll begin with the second one: the answer is partly because they're human and partly because they don't really know or practise Islamic teachings of forbearance, patience and restraint. There is no excuse for loud, rash, violent, destructive behavior in Islam. So honestly, these are just examples of Muslims with no forbearance.
For the first question, I know it can be hard to understand in the 21st century in a society that views religion as backward human-made relics from another era. If that's what Islam was, I wouldn't think much of it either. A Muslim however believes that an almighty being created them and everything else in the universe. And that this being set out certain guidelines for our temporary life in this world. We love our Creator and we try to get as close to Him as possible. We love the Messenger that He sent to us with His Message and who lived an exemplary life. Muslims are taught to love Allah (the Creator I mentioned a few sentences ago) and His Messengers (Muhammad (saws) in particular) more than anything else. Muslims try to live their life by Islam - it occupies a high place in their thoughts and actions. So hopefully this explains why Muslims are so passionate about their religion. It's the same concept as with anything any human loves. Everyone loves their parents, and would be understandably upset if someone attacked or insulted or did anything to their parents.
Coming back to the second question, like I said earlier, there is no excuse and I'm not going to try to guess or justify why those people did what they did. It is clearly not the Islamic way of resolving things. For Muslims, some parts of the Prophet (peace be upon him)'s life is taken as a source of legislation. His forbearance, patience, justice, etc. fall within that part. The Prophet (peace be upon him) never once reacted with violence even when he was faced with far worse insults and physical attacks. He never had a harsh word for his fiercest enemies. He never fell to the level of those who opposed him. Ask any person who knows even a little bit about Islam and they will tell you that this is the behavior Muslims are supposed to abide by.
Allah (our Creator) said in His Word,
And the servants of the Most Merciful are those who walk upon the earth easily, and when the ignorant address them [harshly], they say [words of] peace
The Prophet of Allah, Muhammad (peace be upon him) said:
Allah is the Most Forbearing, and He loves forbearance, and rewards for forbearance while He does not reward severity, and does not give for any thing besides it (forbearance)."
And there are many, many, many more proofs from the primary texts for this. The Prophet (peace be upon him) is known to have said that the believers who have the most perfect faith are those that have the best behavior, and the best of you are those who are best to their wives. He is also known to have said that Allah loves noble character, that there is nothing heavier than good character on the scale of deeds. One time a man came to him for advice, and three times the Prophet (peace be upon him) told him: Don't get angry!. And Allah commands His servants to say that which is best, most perfect, most righteous, most beautiful.
Islam is a constant struggle - it came to transform humanity. To transform them from being a slave to other things like their own whims and desires to worshipping their Lord. Clearly someone people are not very far along in that transformation and still fall into base human desires of violence and loud talk to protect something they value.