at least with me, anyway. so here goes.
there are a variety of tacks a love letter might take, and a diverse collection of possible content which might appear in a love letter, no? (my examples would be the love letters from love's labour's lost and merry wives of windsor (beneath the cut), or sonnet xviii).
so can it really be a coincidence that the love letter we have from hamlet to ophelia reads:
doubt thou the stars are fire;
doubt that the sun doth move;
doubt truth to be a liar;
but never doubt i love.
the love letter could've been an ode to ophelia's virtues, or a "say you'll be mine" imploration for a commitment, or it might've been about how vast his love was ("my love is like ... whoa!" as the song goes), but instead it's about doubt and belief.
isn't shakespeare here giving a direct signal to the audience? "hey, look, don't doubt hamlet's love for ophelia. you're going to have reason to do so soon enough, but never should you doubt it."
that hamlet's love letter tells ophelia to never doubt he loves her is:
'By heaven, that thou art fair, is most infallible;
true, that thou art beauteous; truth itself, that
thou art lovely. More fairer than fair, beautiful
than beauteous, truer than truth itself, have
commiseration on thy heroical vassal! The
magnanimous and most illustrate king Cophetua set
eye upon the pernicious and indubitate beggar
Zenelophon; and he it was that might rightly say,
Veni, vidi, vici; which to annothanize in the
vulgar,--O base and obscure vulgar!--videlicet, He
came, saw, and overcame: he came, one; saw two;
overcame, three. Who came? the king: why did he
come? to see: why did he see? to overcome: to
whom came he? to the beggar: what saw he? the
beggar: who overcame he? the beggar. The
conclusion is victory: on whose side? the king's.
The captive is enriched: on whose side? the
beggar's. The catastrophe is a nuptial: on whose
side? the king's: no, on both in one, or one in
both. I am the king; for so stands the comparison:
thou the beggar; for so witnesseth thy lowliness.
Shall I command thy love? I may: shall I enforce
thy love? I could: shall I entreat thy love? I
will. What shalt thou exchange for rags? robes;
for tittles? titles; for thyself? me. Thus,
expecting thy reply, I profane my lips on thy foot,
my eyes on thy picture. and my heart on thy every
part. Thine, in the dearest design of industry,
DON ADRIANO DE ARMADO.'
'Ask me no reason why I love you; for though
Love use Reason for his physician, he admits him
not for his counsellor. You are not young, no more
am I; go to then, there's sympathy: you are merry,
so am I; ha, ha! then there's more sympathy: you
love sack, and so do I; would you desire better
sympathy? Let it suffice thee, Mistress Page,--at
the least, if the love of soldier can suffice,--
that I love thee. I will not say, pity me; 'tis
not a soldier-like phrase: but I say, love me. By me,
Thine own true knight,
By day or night,
Or any kind of light,
With all his might
For thee to fight, JOHN FALSTAFF'