Sometimes , with V3's, we think they die for no reason when in fact we have forgotten to check the heart meter after certain events.
For example, certain items such as the chest can empty all the hearts (if it has turned the tama into a baby for a few seconds). If they are not checked straight away and so not filled back up - the tama will die- even if all the hearts were full just before using those items.
I have also noticed that the V3 will lose hearts when connecting with a different version tama- even if it wins games- or has had a visit- so after maybe 10 connections with a different version it will have lost all it's happy hearts.
Just a couple of things like this can affect your tama's happiness, even though you haven't actually neglected it at all!