Hi I studied Yeats this year and I found it fascinating how the themes like the one's you mentioned ie: love, aging etc can be applied across a broad range of social contexts and can be read using many different frameworks eg:Romaniticism.
I think the key point to understanding why his poems are worthy of critical study is recognising how universal his themes are. ie: they relate to all of mankind. read through his poems and think about this. Eg: we all age and we all experience love. Thus his poems are relevant to all social contexts. Also try to mention how his 'themes' can be intepreted in different/ contrasting ways eg: a modernist reading may intepret the line "a terrible beauty is born" (Eatser, 1916) as a breakdown in society whilst a nationalist reading may interpret it as a promotion of te beauty and unity attained through national unity and independence.
I hope this has helped.
