People generally get along quite well (excluding certain regions of the world obviously) but I do find both sides are exceedingly hostile with one another on the Internet.

In fact, in my experience the religious are more aggressive Irl than atheists but, for whatever reason, it always seems like atheists are the more hostile and "rain on your parade" online... Can't tell you how many times I've seen someone say something along the lines of "stay strong, I'll pray for you" at the bottom of an article to be immediately followed by "so talking to yourself is going to help?" or something akin to that.

To answer the question though, the reasons people won't get along:

1. You're always right
2. Everyone who disagrees is wrong
3. Some people are just assholes
4. Teenagers/first year college students know more than you

I'm sorry, I don't know what you mean with "political religious" =p

Lack of Understanding. On both sides. If you don't have understanding, how the hell can you have tolerance. Personally, I am a Christian, and my faith instructs me to profess the gospel. However, I also understand people who aren't privy to it.

You can't force a horse to drink for a stream.

You talking about the people over 50 right?

Cause most younger Catholics like me are open to it and believe everyone has their rights

Because people on both sides are very judgmental, because people don't like it when folks have viewpoints that counter their own, and therefore; people want to convert each other.

Obviously it can get rather irritating if they're insistent on converting you, but I always try to look at it from their point of view and be patient whenever someone knocks on my door. From their PoV they're putting time aside to do something that is not fun and quite awkward as they believe they're helping both themselves and the person they're talking to in a way that nothing tangible can.

Of course if it's the type that point a finger in your face and go all Dante Alighieri on you I tend to lose patience, but most are generally very nice people. 

I only have a problem with the ones who try to push their beliefs on others and the ones that throw objective thinking right out the window if it goes against their faith. The world is not black and white and you're a fool if you view it as such.

Exactly.

You know what grinds my gear. When people use religion as a scapegoat for humanities problems. No. It's people who've caused all the problems, all throughout history people have used "ideals" to justify terrible actions. The crusades, the nazis, etc. People carried those actions out, not belief, not ideas or mentalities. Nobody wants to take responsibility, so they scapegoated then and they scapegoat now. 

I don't have problems with most religious people in my personal environment. Even though I despise some traditions (Halal slaughter), complaints about the right to not eat or drink during a certain period.

But silence is golden for all I care, so most atheists and religious people don't hate eachother, especially Western Christians have seen to accept Atheism and science as legitimate knowledge. Whereas other religions would like to claim they have the right not to be educated about evolution and still denie the holocaust and disrespect the (mostly female) teachers who try to educate them. But that's luckily a small minority.

I do believe the 20th century (Stalin, Mao, Pol Pot, two world wars, cold war and so forth) adequately demonstrated that we don't need religion to find sufficient reason to kill ourselves.