It was below 1k last year beginning. But from the mid year prices started to explode! What's the reason behind these price rise?
Bitcoin has high price because the demand for it is very high. Normally for anything which has high demand has high price. Not only the demand, the popularity, technology and usage drives this crypto price higher and higher every day. As you said Bitcoin price is high but as of now the price is low compared to the last year high price it achieved.
At the moment when purchasing fiat money for bitcoin, you always receive bitcoin worth of the value of the money you spend. Hence, for the purchaser the bitcoin price in relation to what he receives for his fiat money is always the same.
If the bitcoin price depends on the demand as you claim, why did the demand suddenly decrease for 50% in January this year?
The price of a limited product increases with the increasing demand. But crypto currencies are in general not limited. A certain crypto coin is limited, but not crypto currencies as such. And other crypto coins also fulfill their task and there is no need to buy the famous ones.