Happiness is dependent on duration: it can only appear to exist in the sequence of 'time'. Moreover nobody can know that he is happy - an animal doesn't, a child doesn't; a man may know it afterwards. Therefore happiness can only be an effect of memory.
You look at an animal, a child, a man, and you say 'he is happy'? It is you who see it: he doesn't. You may be right, but it is you who recognise whatever you mean by 'happiness': what you recognise exists in you, and nowhere else wherever. Moreover in you also it only appears to exist in relation to memory - memories of memories of something you never knew otherwise at all.
You say that you can train yourself to recognise it almost at once? Almost - but not quite, for even then it belongs to the 'past'. In order to know what it is you cannot any longer be subject to the passage of 'time', which means that you can no longer be 'you', and that 'it' cannot any longer be 'happiness' - for then, what it is - you must be.
It never existed at all: it is merely your interpretation from memory of your own intemporal nature.
I am: it is you who supply the details - and they are whatever your reactions may imagine, but they belong to 'you' and not to 'me' - for there is none such, other than in your mind.
Note: When a dog is released from a rabbit-trap in which his paw has been caught, he bounces with delight? When you win a bet at long odds, or receive any unexpected satisfaction, you also bounce with delight? Quite so. That proves, if proof were necessary, that all counterparts are mutually dependent, more spectacularly revealed in cases of sudden contrast, but experienced after all departures from a norm. However long or short its duration in a time sequence, it is always a memory that you 'enjoy', never the event as it is occurring.
In myself I am nothing, exactly no thing: I am only a mirror in which others see aspects of themselves and attribute the resulting concepts to 'me'. But I am also an 'other' to my 'self'.