How to understand flavors and when to use combination of them?
Yes, all dishes should have flavour combinations, unless the dish consists of one single ingredient with no seasoning added, no oil added, it can't help but have them... even then a tomato for example has different flavour in the skin than it has in the pulp, than it has in the seeds, the inner leaves of a brussels sprouts will have less bitterness and more sweetness than its darker outer leaves... flavour combinations are almost impossible to avoid.

So to balance flavours you do have to understand them. Not everyone might agree with the combinations you choose to create and not everyone enjoys the same taste combinations, but even so, to reliably create the blend you do like, you need to understand how it is composed.

There are any number of resources online which will break flavours down to a few key groups, though not everyone describes it the same way. Essentially you need to understand what is meant by basic terms such as sweet, bitter, sour, umami and salt. To those five you can also add 'spiciness/heat' which is often considered to be more sensation than flavour.

You should read up on what it already accepted knowledge about the effects these flavours have on each other, how salt changes perception of bitterness, how sweetness can counteract excess salt but leave the umami clear. Read up what chefs have to say about these interactions and test them out, see if you detect the same effects they do. 

Spend time tasting your ingredients and training your palate (I was so busy thinking of flavours as a palette from which one can choose the equivalent of colours to paint a dish as you might paint a picture that I originally spelled 'palate' as 'palette') so that you can analyse a dish and detect what makes the difference between a combination you like and one you don't. Understand your ingredients, both fresh and storecupboard ones well enough to know quickly what will make that difference.