Deep question...
But I often struggle with this...
How do you know for sure that you love someone? Be it, a parent, child, partner, wife etc...
How can one know 100% that what they feel is love, baring in mind that 'love' has no standard definition, and you can't possibly know what other people feel.
If you say yes you feel love, how can anyone know that the 'love' they feel which is subjective in the first place, is the same feeling and intensity as what other people feel.