We love our spouse, parents, siblings, children, friends, country, and many more things.

But, Why do we love someone or something? One of the reasons is the connection from past karma, but in this life, why do you love someone?

Brihadaranyaka Upanishad explains this very nicely.
स होवाच: न वा अरे पत्युः कामाय पतिः प्रियो भवति, आत्मनस्तु कामाय पतिः प्रियो भवति । 
न वा अरे जायायै कामाय जाया प्रिया भवति, आत्मनस्तु कामाय जाया प्रिया भवति । (2.4.5)
Wife loves husband not for his sake, but wife loves husband because loving husband helps her love herself. Similarly, husband loves wife, not for her sake, but he loves wife because loving wife helps him love himself.

In short, we love others because we love ourselves, and loving others makes us happy, that’s why we love others.

This is very evident from the fact that both husband and wife, try to change each other to make them more suitable —> more suitable to please their own self (it’s secondary if that makes the other person happy or not).

The same is true for every relation, every object, every possession in life. They all are there because we think they make us happy, and we want to make ourselves happy as we love ourselves the most.

But, why? why do we love ourselves so much and want to make ourselves happy?

In order to answer that, we need to take help from Taittriya Upanishad.
आनन्दो ब्रह्मेति व्यजानात्। आनन्दाद्ध्येव खल्विमानि भूतानि जायन्ते। आनन्देन जातानि जीवन्ति। आनन्दं प्रयन्त्यभिसंविशन्तीति। (3.6.1)
Our true nature is Happiness/Bliss; Happiness/Bliss is where we are born and it is the thing which we live for, it is where we go at the end.

So, to answer the question, we love ourselves and want to make ourselves happy because, “Bliss/Happiness” is what we are made of, so we try to collect things around us which can make us happy.

But, do these people, things really make us happy? Oh, that’s a different matter.. but, we try because we love ourselves immensely.