hmm...

1. Spike sees Twilight as a Mother figure, or Guardian

2. Spike sees Twilight as an older Sister

3. Spike was hatched with Twilight's Magic

3a. Spike might be "magically" related to Twilight

3ai. Twilight might "magically" be Spike's Mother

3b. Spike might be "magically" part pony

3c. Evidence - All other dragons have wings

3d. Alternate theory - Spike isn't actually a Dragon

4. Spike is a "baby" dragon

4a. it's common for Mothers to wash or bathe with their Babies

4b. it's common for older siblings to help bathe infant siblings

5. Spike is attracted to Ponies

5a. Spike has long had feelings for the Unicorn Rarity.

5b. Spike could theoretically develop romantic feelings towards Twilight too

5c. depending on how Spike acts on these feelings, his actions may appear creepy or weird.

6. As Spike is a dragon he'll live a very long time, practically immortal.

6a. Twilight as an Alicorn, may also now be practically immortal

6b. it's logical for an immortal to form a courtship, with another immortal.

6c. they'll both need someone to provide comfort, when all their mortal friends and family Die.



but then it all depends on how Twilight decides to react to this situation