According to some religions and philosophers/theologians, the existence of God follows logically because there must be someone to create the universe. (The first cause argument.) 

However, in many of these religions, there's an assumption that this God is loving. **How did the assumption that he is loving originate?** At best we can say that he is neutral but I am unable to find any solid reasoning for his love. 

I'm curious about Sikhism in specific but also interested in other religions that have the same assumption.

As asked by @JoWehler, here are some references to how Sikhism describes God. From *Japji Sahib pauri 5*:
> thaapi-aa na jaa-ay keetaa na ho-ay. aapay aap niranjan so-ay.<br/>
He cannot be established; By doing it cannot be done. He Himself is Immaculate and Pure.

Again from *pauri 5* :

>jin sayvi-aa tin paa-i-aa maan. naanak gaavee-ai gunee niDhaan.<br/>
Whoever serves Him receives honor. O Nanak sing – “Only Lord has endless Virtues”.

Here they call God pure and virtuous. Other religions make similar claims. How have theologians argued that God is loving/good?