Why upgrade?

I have a Nexus 4 and will have no intention on upgrading for quite some time (maybe another year or two). With phone specs almost equalling that of a cheap laptop, why upgrade? Your phone can only run so fast. Are 13MP pictures really that important to you compared to 8MP pics? Is a 328 PPI screen that much better than a 327 PPI screen? I'm pretty sure you don't buy a brand new car every year the new model is out because cars are meant to last, same with phones.

So back to my original question...if you have a phone released from the past 12 months and are planning to upgrade within the next 3 months, why upgrade?