And we find out in the s10 comics that when Spike thinks of or looks at Buffy he sees her glowing in that “lights dim everywhere else” type of way. So the fact that he tried to keep her in the dark, told her she is a creature of the darkness similar to a soulless vampire, said she belongs in the dark, that is really screwed up. He wanted her as his personal night light, shining just for him.
I love that Buffy saw Angel as, dare I say it, effulgent. She believed he deserved to be in the light too. That he had light in him.
And we know Angel always wanted her to be able to walk in the sunlight and be bathed in what to a Catholic souled vampire must be God’s something or other since there is so much association between demons and evil beings barred from holy places because they are unworthy and not forgiven or saved. Darla shoving a cross in his face and saying “God doesn’t want you!”
The very first time Angel sees Buffy she’s in the sunlight. He dreamt of marrying her in a cathedral and walking hands held in the sun. When he introduced himself to her he gave her a cross necklace and told her “don’t turn your back on this.” Angel knows Buffy can for the most part take care of herself. It was her heart and soul he wanted to protect.